Welcome everyone to my 2nd build (1st is this one, because it got deleted and im making it again...), hope you will enjoy it and found it usefull.

Note to new patch update: Yes, AD Katarina was nerfed in last patch and AP Kat was buffed, so that means AD = AP (not like AD > AP before) Death Lotus's basic damage was nerfed for 20-25% but still its pretty powerfull. So Kat is now more a hybrid champ benefiting from AD and AP will be the best way to go, but still i prefer more AD. ;)
-> 31/12/2010 - item build now up to date to this patch
-> this isnt the AD build anymore, its more like hybrid, because its works better then pure AD or AP

Katarina, the Sinister Blade

To the start, dont downrate, because you dont like this champ or AD build for she as overall. But comment and rate, so i can see the feedback and improve.

Yet, this is another of those "hard to master" champs, so dont downrate because you wont get those 30-2-x scores right from the beginning.

Shortcuts
CC - Crowd Control
BB - Bouncing Blade
KI - Preparation
Ult - Death Lotus
Juke - Escape from "death grasp"


PROS
- one of the best harassers
- very good burst damage
- excellent finisher or clean up
- Death Lotus
- easy jukes
- dependable at your timing and reflex skills
- sexy look and voice ;)

CONS
- squishy
- no CC's and dependable on enemy/your team CC's
- hard to master
- dependable at your timing and reflex skills


MASTERIES
For here i take little non standard 21-9-0. Brute Force will give you little boost in early game for the harassment and rest is probably self telling, so this doesnt need more comments i hope.

Yes i seen Katarinas with 9-21-0 or 9-0-21, but i found the utility one (9-0-21) kinda useless, because iam addicted to massive damage and to the defence one (9-21-0), well, its not that bad as it looks, give you little more survival early on, but i like the damage and fast early kills... :)


SKILLS
Voracity
This is why you are one of the best finishers/clean ups and how you will deal extreme burts damage in teamfights. Your BB and Shunpo will get refreshed right away after kill/assist, meaning so you can Shunpo+BB to another, another and another targets. And if you are alive, then use your ult 2 times in 1 teamfight.

Bouncing Blade
Main ability to harass, finish running opponents and deal massive damage in combination with KI. You can also use it to last hitting minions or just killing whole wave in later game. But my game is kinda passive and agressive both at a time, ill get to that later, but also its my reason to level this up 1st, because it will deal more damage then Shunpo and you will be save most of the time.

Preparation
One point to this ability is all you need. Just level it up last, because other levels arent so good to take, but in mid/late game it is very powerfull in combination with BB to get 100% damage in all bounces or with Shunpo to get a little survival while channeling your Ult.

Shunpo
Instant teleport, moving you behind your target and dealing damage. I personaly level it up as 2nd, because you are not AP Katarina, so you wont do epic damage early on and i dont play so agressive. Well, use it in combination with BB + Ignite to finish enemy early game.

Death Lotus
This is your "carry eater", sometimes "whole team eater" and also your biggest weakness. Why? Because its channeling that means so any CC (snare, stun, air knock, silance) will interrupt this skill, making you useless for next seconds until your abilitys will recharge. This skill works best, if all enemy CC's are wasted at your team and then you just Shunpo in middle of the fight and get the kills, its only about timing. Also you cant move while channeling (if you do or if you use any other skill, your ult will be interrupted), but you can use your summoner spells like Ignite Exhaust Rally or Heal while channeling it.


RUNES
Marks
Greater Mark of Magic Penetration, yes, magic pen. because all of your attacks deal magic damage, even if you are building AD items. But its also good thing to have crit chance runes if enemy team have a lot of CC's.

Seals
Greater Seal of Evasion as you seen above, you are squishy and dps carrys like Jax, Tryn or Yi will just eat you. Thats why to take them with Nimbleness . Well, if you feel unlucky or you doesnt like chance of anything, then go for flat health or health per level runes.

Glyphs
Greater Glyph of Cooldown Reduction well, you are an cooldown based champ, so take these runes to get a little advantage in these "1 sec cooldown left" situations.

Quintessences
Greater Quintessence of Health and again, you are squishy. To counter it, just use flat health. More health, means more time at a line, more harassment, more kills/assists and sooner tower dive capability.


SUMMONER SPELLS
Which to use.
Ignite - Very good for finishing opponents in early/mid game.
Exhaust - Slow, lower magic resist, armor and blind. This counter your lack of slows and those anoying dps carrys.
Ghost - Get you faster in or out fights, great for chasing down runners and very good for tower dives.
Rally - Dont be fooled. This will give you a ton of damage and ability power for your Ult, helps your team's damage output and it will eat tanks. Only pain in the a*s is the cooldown and your timing.
Heal - Take this if you are new with Katarina or you play very aggresive to get that upper hand in early kills and later teamfights.

Consider before trying.
Flash - Double flash Katarina. Not bad, but its useless while tower diving, because of your passive (Shunpo in, kill, Shunpo out), but its only matter of opinion and awesomeness (yes it looks good).
Cleanse - If you are having a problem with CC's, which you shoudnt have and also good while you are being chased by slowing or short cd stunners. But i personaly dont like it.
Smite - Yes, thare is jungle Katarina build. But i found myself more usefull at a line.
Teleport - Good for pushing/defending towers, fast swiching lines, getting back, faster recall in crysis or while chasing someone.

No way!
Revive - No, everyone knows why.
Clarity - You dont have a mana, right?
Clairvoyance - Good for supports... You are not a support...
Fortify - Same as Clai and i personaly found it useless.
Promote - No, because you are not level 31 to pick that. And with minor problem so it doesnt exist. ;)

But be aware... Ignite > Exhaust in early game... Ignite < Exhaust in mid/late game.


ITEMS
This is the tricky part. I cant tell you define build, with which you will be getting these penta kills, which one will suit your play style and how much CC's will have your enemy team, thats why im gonna show more of them.

Core items
Hextech Blade

Detailed
- yes, its better then after the new patch with +8 damage, meybe its not look as good as you want, but this is very good for BB in early game

- to get the upper hand you need in these fair fights, meybe this is all you need to get in the lead

- magic penetration for your damage output, because all your skills deal magic damage or magic resist and -35% duration of CC's if enemy got a lot of em (3 or more) or dodge (now around 20%) if thare is a lot of dps carrys (2-3 or more)

- if you get 2 or more kills before you are going for the 1st shoping after laning, then buy it +110AD and +15% movement speed after few minutes is worth it, later on you can sell it and buy another Bloodthirster
-> same thing if you'd like to take Mejai's Soulstealer, but its worth a price only if you get 5+ stacks at it, so if you getting focused, buy somethinig else

- get this asap after the boots and get max stacks on it, aslo yes, its good idea to have more of them, because theyr passive isnt unique which means in case you will have warmog, shoes, 4Bloods, then you will get +4AD per 1 minion kill, maxing at +160AD

- only if in game is a lot of enemy CC's and if they are clever, they will wait for your ult and then CC you to interrupt, therefor get this one instead of 1st Bloodthirster to get a little non-obvious ult only Katarina and get a crit after Shunpo as a suprise

- yes, i know, this is AD build, but slows are always good, get this as last item to ensure nobody will get away from you and get a little more survival, good for early/mid game

- if you are feeded, you can take this, for the magic penetration, especialy for users which dislike Rylai, also this will help your damage output (obviously), so its good if you are not being focused or if you are feeded in way so you just kill everything and they just wont have a time to react

- if you are being focused, get Warmog instead of Rylai after 1st Bloodthirster to help a lot your survival, which will stack up along with your damage

- and alternative of Rylai or Warmog if you got clever CC user against you, this will interrupt the 1st spell cast on you every 30seconds, so you can Shunpo in, Death Lotus and it will take 2 CC's to interrupt you, plus a little survival

- well if you are not favor of Rylai to slow with skills and you got a jungler in the team or you doesnt want to jungle, you can take this for a survival and slows with normal attack and that bonus in form of 20 AD

- good item for survival early game, you can sell it upon getting 1st Bloodthirster or second BF Sword

Hextech Blade - now its one of the best items for Kats, because it gives both damage attributes you need and survival (life steal and spell vamp) + slow is always good

note to: - its not worth the money, because its passive is useless to you in a simple way so your major damage output isnt physical, so its a better to take another Bloodthirster or Infinity Edge if enemy is full of CC's


Yes i know, its a lot of items, but you must adapt to enemy, to counter his champ abilitys, make them useless and kill them all. ;)


TIPS
1) dont never, ever die, if you are getting outharassed buy survival items - start Warmog, but death is not an option, even in trades 1 for 1, its better to leave him get back and you will have time to push the tower or gang other lanes and your death can make your team lose in late game, so dont try the fortune

2) harass, harass, harass, you are one of the best harassers in the game without skill shots, so get them under half health and if they wont back, learn them a lesson of how squishy they are

3) map control is half of sucess, use the Sight Wards to avoid ganks and never die

4) little to jukes... well with Shunpo you can teleport at wards, Heime turrets or traps, use it while planning your escape route... also you can enter a bush, wait until enemy gets close and when he enteres the bush, then go the way he came from, if hes bad, then he wont use autoattack move and you are a little closer to your immortality or eventialy if more people are chasing you, then you can Shunpo on top of the last one and go the way opposing of his... in other words dont be predictible and dont use same escape route again and again

5) for early kills, is very good combo Shunpo + Bouncing Blade + Ignite and if enemy isnt in range, just Shunpo to the minion and get try get a kill only with BB and Ignite

6) dont never, initiate the fights (or you will die), unless its a sure kill, of course dont be stupid with getting a 1 kill in 1v5 :P

7) Ignite Exhaust Rally Heal will not interrupt the Death Lotus's channeling

8) adapt, thats how you will recognise the skilled players, they wont do same builds all the time, they will adapt to the enemy, counter him and then steamroll

9) with who to lane with? its simple, the best works the another damage burster and/or CC user, like LeBlanc, Sion, Sona, Nasus, Kayle, Taric, Akali,...


SKILL ROTATION
1v1/1v2
As you seen above, harass with Bouncing Blade until hes get under 70-40% (depends on total health, armor and magic resist, normaly its somewhare about 500hp in early game), then Shunpo behind a target, move a little to his escape route (95% his tower) and start Death Lotus, Ignite, Exhaust, Rally during your ult to get a kill for 100%, then Shunpo + Bouncing Blade to another target, if thare is any and you arent close to death - use Preparation depending on your enemy, if he got heals, reduce it to 50%, if not, take it with survival bonus

Teamfights - 4v4/5v5
Here is the only this which matters to you. Timing. Harass until you can and wait until enemy get his CC's on cooldown, then jump in the fight with Shunpo and eat them all with Death Lotus, if you get focused during the channeling, its better to start the burst damage even if your ult isnt finished (lets say so 1sec remainig until the end) - by burst damage i mean combo of Bouncing Blade + Preparation + Shunpo and repeat to all targets immediatly you get an assist or kill, thats why you are the clean up


SCREEN for your happiness ;)note: you can see defeat in 3v3 down thare, i tryed AP Kat and i just suck with it :)
note2: if the image apears to be small, use CTRL + Mouse Wheel to zoom in

Items, masteries, runes and summoner spells are dependable on combination of these 4 categories. So its not 100% how i play/build Katarina.

Well thats probably all. I hope this is/was helpfull to you and post your comments below.
Thanks to Furor for his guide, from whare i taken an inspiration and from whare i learned how to play Katarina.