Caitlyn is my main champion at the moment. I prefer caitlyn to any other AD carry (ashe, mf, vayne) just because she has 650 range, a finisher ulti(no one gets away with 20 hp NEVER), a Q ability that deals a ton of damage both early and late game.
I love caitlyn because thanks to your 650 range you can hit everyone but if you position yourself correctly no one will be able to hit you. A lot similar to tristana but tristana power is only massive late game while caitlyn can dominate early game ESPECIALLY IN A LANE LIKE CAIT+TARIC that lane is just devastating to any other.

The runes are a bit different than what ive seen but i will explain.
REDS- reds are standard :p
YELLOWS- i take flat armor yellows because in bot lane if you and the other carry start trading blows you will win unless they have a soraka that has her heal of cooldown of course.
BLUES- i find attack speed blues fine, but if you are more confortable with it you can exchange it for cooldown reduction which is always good.

Standard carry masteries with the magic penetration because of your ultimate.

About items AD carry have expensive builds but thats why you go with support bot lane to farm non-stop.
Start the game grab a DORANS BLADE, next recall 2 more DORANS BLADE and BOOTS. Then farm your *** off to get money for a BF SWORD and BERSEKER GREAVES. If you get these items before the other ad carry does you will just dominate his butt. Then get THE BLOODTHRISTER, i choose to rush a bloodthrister because at that point you can farm really easily and fast the creeps to give you attack damage since i almost never die (example my last ranked cait game 12/0/7) if u dont die it means that you have a 40 AD steroid which is awesome. Then rush BLACK CLEAVER to right click tanks to death :D. After that i build a zeal to give me a move speed boost because you are very slow... Then FROZEN MALLET to make sure that no one escapes you (your team) It is EXTREMELY helpful to secure your team kills like that with your 650 range you might be able to slow an enemy that your team wouldnt be able to kill. Then PHANTOM DANCER and INFINITY EDGE to just crit everyone and dominate lategame.

I max Q first then E because i like the cooldown reduction on E to escape and to increase your early damage a bit.
I've gotten many kills by hitting an enemy with E and then ultying from a distance.

Exhaust to negate any xin, yi, jax or trynda that dives you in teamfights or even the other ad carry.
Exhaust is an awesome summoner spell like flash so just go with that.
*OPTIONAL*
the only other option i see is Ghost Flash also works very well.

Cait role in the game is to RIGHT-CLICK its very easy to play caitlyn but i see so many people having trouble playing her for some reason :S. Just dont give her up and you will see that she's very rewarding.