One of the reasons Piltover is known as the City of Progress is because it has an extraordinarily low crime rate. This hasn't always been the case; brigands and thieves of all sorts used to find the city-state an ideal mark for plunder, primarily due to the valuable resources it imports to fuel its techmaturgical research. Some even theorize that it would have fallen long ago to the chaos of organized crime if not for Caitlyn, the Sheriff of Piltover. Born the daughter of a wealthy statesman and a pioneering hextech researcher, Caitlyn discovered her natural gift for investigation when, at age 14, her father was assaulted and robbed on his way home. She snuck out of her house that night with her father's rifle and tracked down the muggers from the crime scene. At first, her parents did their best to discourage her from such risky hobbies, but she was incorrigible. Wishing to protect her daughter in the only way she knew how, Caitlyn's mother began outfitting her with techmaturgical devices tailored to her sleuthing needs.
Caitlyn quickly gained notoriety, both because she was singlehandedly defeating crime in Piltover and also because she soon developed into a ravishing beauty. She never backed down from a case or a challenge, and she was one of the sharpest shots in the city-state. Her services were soon requested by Demacia to help track down a mysterious outlaw who had begun committing high-profile heists. The bandit, who always left a card with an ornate "C" at the scene of the crime, became Caitlyn's arch-nemesis. To this day, Caitlyn still searches for this cat burglar, and the chase has led her all across Valoran. She has joined the League to hone her skills and gain the influence necessary to track down the only quarry that has managed to evade her.

"Go ahead, run. I'll give you a five minute head start." –- Caitlyn, from her book Willing Apprehension

Pros:
Massive Damage Output
Great Range
Fairly good escapability

Cons:
Squishy
Often focused
Can be countered easily (Thornmail)

The marks of desolation and the quints are necessary, but the attack speed glyphs and seals can be changed to either attack damage or health, depending on your play style.

Here's where things really can make or break a game. Rushing the 2 Doran's Blades(eventually selling to make more room), Beserker's Greaves, Infinity Edge, and Phantom Dancer are necessary, but afterward, depending on the enemy team, you have to play it smart. The build shown is if you are doing work and don't really need anything but more damage.

For Getting focused too quickly (sustainability):
Frozen Mallet in place of Bloodrazor
Atma's Impaler in place of Black Cleaver (This should give you ~50 AD with your Frozen Mallet)

For Prolonged Team Fights:

Stark's Fervor in place of Bloodrazor NOTE: this will not only help you, but any other ad teammates that need lifesteal.

Last Whisper in place of Black Cleaver to take down the tank.

Losing against AP heavy team:
Ditch the offense Black Cleaver and go for a Banshee's Veil. Not only will it give you MR, but the mana and health is helpful

Losing against AD heavy team:
Picking up a cheap Thornmail could be devastating on top of your normal attacks.

The skills are basic: Max Piltover ASAP and alternate leveling between 90 cal net and traps, taking Ult when possible.

(Active): Caitlyn revs up her rifle for 1 second to unleash a penetrating shot in a line which will deal physical damage to all targets hits. It will deal 15% less damage for each subsequent target hit, down to a minimum of 40% damage dealt.
Range: 1300
Cost: 50 / 60 / 70 / 80 / 90 mana
Cooldown: 10 / 9 / 8 / 7 / 6 seconds
Physical Damage: 20 / 65 / 110 / 155 / 200 (+1.3 per attack damage)


Yordle Snap Trap (Active): Caitlyn sets a trap at the target nearby location. The traps triggers when a champion walks over it. When sprung, the trap immobilizes the champion for 1.5 seconds, dealing magic damage over the duration and additionally revealing the target for 9 seconds. Caitlyn can set up to 3 traps and they last 4 minutes. When she sets a trap once the cap has been reached, the oldest trap will deactivate itself.
Cost: 50 mana
Placement Range: 800
Activation Range: 150
Cooldown: 20 / 17 / 14 / 11 / 8 seconds
Magical Damage: 80 / 130 / 180 / 230 / 280 (+0.6 per ability power)

90 Caliber Net (Active): Caitlyn fires a heavy net in front of her. The net will slow down the first target hit by 50% and will deal magic damage to it. Upon firing it, the recoil will knock Caitlyn back.
Cost: 75 mana
Range: 1000
Knockback Distance: 400
Cooldown: 18 / 16 / 14 / 12 / 10 seconds
Total Magic Damage: 80 / 130 / 180 / 230 / 280 (+0.8 per ability power)
Slow Duration: 1 / 1.25 / 1.5 / 1.75 / 2 seconds


Ace in the Hole (Active): Caitlyn marks an enemy champion at a huge range and channels for 2 seconds to line up the perfect shot, providing vision of the target for the duration. She then fires the projectile to deal massive physical damage. Enemy champions can intercept the bullet for their ally.
Cost: 100 mana
Projectile Speed: 3200
Cooldown: 90 / 75 / 60 seconds
Physical Damage: 250 / 475 / 700 (+2.0 per bonus attack damage)
Range: 1900 / 2050 / 2200

Flash/Exhaust are typical AD carry spells, but I've found success with both Teleport(for helping team in lane phase and side-pushing) and Ignite(for those enemy healers or to finish someone off)

Make sure you are far behind your team when initiating. Your range will keep your damage in the teamfight, but prevent you from getting focused. When focus does turn to you, flee for your life, and, depending on your health, run back in to help the team finish.