Tristana is a ranged carry. She excels at mid and can harass very well while keeping a last hit going. Her passive increases her range every time she levels meaning once she hits level 18 or she can fire away at people at great distances (including from the other side of walls). Her E spell is great farming, you'll clear through herds of minions in a few shots with E maxed out. Her W is well used with auto-cast (Point mouse to where you want to land, Alt > Click, using this method you can always quickly jump to your maximum distance). She can snipe characters down that are tower hugging while still standing outside of the tower's range. She can protect towers from groups of enemies and protects carries with her ulti.
So next time you're at the champ selection screen and want to try out a new character, give Trist a try, I promise she won't let you down.