This build came about when I noticed that Caitlyn's passive ability "headshot" can critical. That means that her regular attacks can deal a massive amount of damage to minions and enemy champions alike. At full build you should be "criting" for about 1400-2400 (every sixth attack or so [earlier in grass!]), depending on what items you feel like getting and when.

Why no desolation runes?! Well, frankly, when you are dishing out that much damage as Caitlyn it doesn't matter if you have armor pen or not. This is because your attack speed runes let you activate you passive faster and faster. The more you headshot, the more damage you deal which will far surpass the damage you would be deal with desolation runes. The down side to this is that your skills are less effective against champs with high armor, but the truth of the matter is her skills are not that great anyways (more on that later.) But, with exhaust+cripple you can deal with high armor champs without having to use all those runes up! Desolation runes give armor pen of about 25 for other builds. Cripple drops their armor by 10. This in my opinion is enough for most any situation unless a champ specializes in armor, like Rammus. Good thing you have only a 1 in 74 chance to see him (and counting). If you do face him build black cleaver!

The greater glyphs of clarity should be all the mana regen you need. If you need more, you are doing something wrong. Be more conservative with Peacemaker, go for shots that will hit two champs and minions, or a lot of minions at once. Like when they approach in a line, that is a great time to peacemaker, it should hit every single minion if aimed right.

All of the masteries are fairly straight forward. I choose greed because the build is very expensive. But the new update saves Caitlyn about 1000 gold! Thanks Riot! The most important masteries to get are Deadliness, Alacrity, Lethality, and Havoc. I put the point in cripple to boost exhaust. I don't put the points in Offensive mastery because minions just die in one hit from her later anyways, i would rather have more cooldown reduction to boost her escape/offensive potential with her 90 caliber net.

Doran's blade... If you really want it then do so. I choose Vampiric Scepter, with her runes making her attack speed outrageous a low level coupled with her passive, you should find you can stay in lane a very...very.. long time. Always keep minions between you and your opponent. Piltover Peacemaker is a good harass but don't be afraid to shoot champs with you auto attack 'hit and run' style. Doing this can help you conserve mana.

As the match goes on the increased attack speed from Berserker grieves keeps Caitlyn's DPS up in lane because of headshot, but you should get B.F. Sword as soon as you have enough gold! Finishing Bloodthirster early will get you plenty of time to build up its attack damage and life steal. And if you die, you will have lots of time to redo the stacks.

Phantom Dancers. By this time team fights should be happening, the increased crit and move speed/attack speed is essential for helping your team and increasing your chase down/escape. Since Caitlyn doesn't have Ashe's slow arrows... Caitlyn needs move speed to be of much use to a team. Above all I would say that Phantom dancers is the most important item to build for Caitlyn. No phantoms = No move speed = death.

Infinty edge is what tips the scale mid to late game. If enemies do not focus you in a team fight, they will regret it very quickly. Caitlyn should be around 2 attacks a second... so every 3 second she will headshot when not in grass. When you have infinity edge, the lifesteal you get from to damage you will be deal can tank A LOT!

After infinity edge build whatever you want... I just put what I like to do if a match goes that long... *cough*

Straight forward as well.
YORDLE SNAP TRAP! Drop that in a team fight if you can, it can change to whole fight. In addition, placing in bushes for anti-gank in very handy. But mastering the Trap offensive capabilities makes Cailtyn lethal. Place traps in the blind spot behind enemy turrets where its impossible to see. You can also place it right at the feet of some champs and that wont be able to see it appearing. (Cho'gath... ) Another good use it to drop it ahead of you as you run from an enemy, you can run over your own trap just fine, and now you enemy cannot follow you in a straight line, making running away much more possible with Caitlyn. Combine that with 90 Calibur net and you should die very little, but you have to nail those skill shots! Don't forget that 90 Caliber Net can make you hop walls as well. The Snap trap is also good for placing at your own feet, enemy champs wont be able to melee you then, since the trap will trigger and stun them. Dumping a few around your turret can deter enemy champs from engaging the tower if placed right. I recommend having two out at all times and keeping a third ready just in case. Note: Place a trap in grass will light all of it up for a brief second, so its great for checking grass areas.
Don't forget 90 Calibur net can launch you forward as well to get closer to an enemy champ!
Not only this, but hitting with the net it launches can save an allies life, even if you personally are not in danger.

Note: Ace in the Hole. Do not just use this to last hit a champ. USE IT more often, it has a short cooldown and does as much damage as Lux's beam. Hitting a carry early can cause them to retreat and make it a 4v5 match up for your team. Also, if your team is backing away, lob an Ace in the Hole, its range is outrageous, and now an enemy champ gets rocked for damage. You have a 99.9% chance to hit with it. -_- ...Sometimes... an enemy can time flash perfect and dodge the shot... (Very difficult to do btw)

Exhaust+Cripple is a must. It increase her offense and escape ability far better than ghost, in addition to greatly helping in a team fight. Ghost... is nice... but and Exhausted enemy carry leads to team wins. A ghosted Caitlyn leads to... Caitlyn getting a kill, but more likely running with her tail between her legs.

Teleport. Although she is not listed as a pusher, she can push lanes incredibly fast. One Peacemaker can clear waves of minions, not to mention with the 250% headshot passive, minions melt, even super minions should take about 4 hits to kill, give or take! Tele-gank is okay... but this is more useful for helping stay in lane and defending towers. With the point in the mastery its also very useful to escape some situations since its faster then "B".

Flash - Meh. She has a few tricks to escape, but if you need more, then grab flash over teleport.

Clarity - If you love spamming her abilities grab clarity... but your autoattacks will be more useful as the game progresses. Mainly, clarity lets you push lanes hard very early game but becomes less and less useful as the game carries on.

Ignite - Great for dealing with Dr. Mundo and helping overall, its a great pick if you don't like teleport. Very useful in team fights as well.

Ghost = Lame in my opinion, it is a very selfish skill compared to many of the others.

Revive - You shouldn't be dying, especially with your range!

Heal - not bad, but heal bait works better for other characters. It does help your team which is always good.

Cleanse - Most of the time you will die too fast to be able to hit the button! Lol. Don't get near enough to be CC'ed or use your net to get away.

Smite - No.

Rally - I guess, its not bad. Anything that helps the team is never bad.

Clairvoyance - This is actually rather handy. Ace in the Hole will sometimes disengage if an enemy gets in a bit of fog, you can use this get a clear sight on your mark. It also has a short cooldown and helps your team a lot. All around not a bad pick if you don't like teleport.

Pros-
Unreal Damage output. (If you are not topping the Damage Chart you are doing something wrong)
Somewhat better build for team fights.
Amazing Pusher.
Can 1v1 much better! (nom nom nom's Ashe)
Longggggggggg Range

Cons-
Very Squishy (Life Steal doesn't work when stunned :/ )
Not the best CC in team fights, good vs. minions
Traps are difficult to use. (Enemies can see them)
Relies heavily on lots of gold.
Thornmail = Bane (build magic resist or you will regret it)
~If an enemy is stacking armor you probably shouldn't be focusing on them in team fights anyways.

Vs. Jarvan IV - wait to use 90 Calibur for when he ulti's, you can hop out of it.
Vs. Teemo - :( His blind ruins Caitlyn, best to wait for your team. Oracle!
Vs. Rammus - Really... don't shot him with autoattack when he has his shield up. Use peacemaker.
Vs. Nunu - 90 Calibur net can get you completely out of his ultimate.
Vs. Ashe - Focus her down, she cannot out damage you, maybe if she wastes her ultimate.
Vs. Shaco - Oracle!
Vs. Evelynn - Oracle!
Vs. Karthus - Get Banshee's veil.
Vs. Tryndamere - Get him low, but don't Ace in the hole, wait for his rage to end, then fire it.
Vs. Blitzcrank - Ace in the hole will not more most of the time because of his shield :(

Blue buff is very nice for Caitlyn, but don't take it if other champs can benefit more.
RED BUFF IS CAITLYN'S ALWAYS. Does not matter if you have a jungler or anything, she has the longest base range every, you give her red buff, always.
In team fights, ALWAYS kill the carry first, or anyone that can stun/fear you. Silence is useless against you, so is slow, for the most part. Taunt is fairly useless against Cait since she was probably Auto attacking anyways...
Again, don't be shy about using your ultimate, if an enemy has to port back that is less XP for them! And less farming!
This should go without saying, but if you are in a team fight near grass... fight from the grass, your passive will love you.
You can solo dragon fairly early (when you get bloodthirster built), and jungle well earlier also.
Caitlyn is best when she is not mid, but soloing bottom lane (near red buff). The grass is much closer to the lane which greatly increases her DPS, and her traps useful ness. She should really be listed as off-carry, pusher, ranged. =P At any rate bottom lane near red is where she belongs, whether solo or 2 in lane.

Always have fun and build to match your opponents!