Caitlyn is the ad carry with the most range, if you count Kog Maw's use of his spell W. This makes her one of the best Ad carries since you can control a lane without being to aggressive and taking to much damage. On top of that she is able to push a lane, and keep pushing a lane more than any other ad carry. She has great disengage and escape tools.

The runes on Caitlyn can vary on the way you like to play the game, but I personally prefer AD runes, with armor and magic resist per level.

I choose flat armor seals, because in the bot lane you need the extra durability, and you need to be able to withstand the harass from opponents.

I choose attack damage marks, because I want the extra damage when I harass and I find it easier to last hit minions with the extra damage from the marks.

I choose magic resist per level glyphs, because I want to be able to withstand the damage from the AP champions, I choose to get them per level because I don't find it likely that I will meet to much AP early game, and after level 9 you will have the same amount as you would have with flat magic resist.

I choose attack damage quints, because as with the marks, I find the extra damage to help me with the last hitting of the creeps, and as you know farming is very important. And on top of that the extra damage will be very nice during the game.

Q over W over E.

I choose to max my Q first, because it helps you push the lane and it will also be your main tool of harass, the more damage you do to your opponent the more likely you are to win the lane.

I choose to max my W second, because your traps will help you control the lane even if they will re-push the lane, and try to make you farm at tower. A few traps in the bush will make them leave it for good, and let you control it.

I choose to ma my E last, because I never use it for additional damage, I only use it to get away or jump over ledges and trough bushes, it is a very important spell to have, but it doesn't mean you have to max it second.

Start: I start with boots and 3 health pots because I want the sustain of the health pots, and I don't think that Caitlyn needs the added damage or health from a Doran's blade.

Always: When I back my first time I'm hoping to fill up my potions, but first and foremost I want to buy a Doran's blade and a Vampiric scepter, since the change in Seasno 3, I find the vampiric scepter to be an important item in every adc build, if possible buy your shoes first upgrade, and get the berserker greaves.

Situational: Then the next items are more situational, when you have the advantage you should buy the bloodthirster for the added sustain and the possibility to stay in lane for a long time. If you are behind you should skip the Bloodthirster and get the Infinity Edge.

End game: I find that most ad carries should use the same items. A full build should consist of: Berserker greaves, Bloodthirster, Infinity Edge, Phantom Dancer, Black Cleaver(since season 3)/Last whisper and Guardian Angel.

Phantom Dancer: This is another situational item, it depends if you have Nunu support, Cait and Nunu has a great synergy, and if you get the constant Blood boil help you can wait with the Phantom dancer until later in the build.

Guardian Angel: In other guides that I used when I started I saw a lot of persons that preferred the Banshee's Veil, I understand that it is a death sentence to get grabbed and engaged on as an Ad Carry, but when playing as Caitlyn you will learn the advantage of range, you will learn to stay in the back of the group and to not rush deep into team fights. And whenever you get focused, the Banshee's Veil won't help you survive, if they want to use everything on you and kill you, you will die. The Guardian will help you get that extra life.

Cleanse:

Flash:

Piltover Peacemaker (Q):

Yordle Snap Trap (W):

90 Caliber Net (E):

Ace in the Hole (R):

Headshot (Passive):