Sniper is one of the most used classes in team fortress, people love using the sniper-rifle to frag in one shot. The reload time for the rifle is real long though, and the sniper has small armor and only takes one rocket to kill him. If you're a sniper, find a good hiding spot with a view of a heavily-traveled area. Also bind your autozoom command ("autozoom") to a key so you can toggle it. Once autozoom is ON, when you hold the attack key the screen will zoom on your crosshairs, allowing you to snipe enemies from far away. But toggling autozoom OFF, brings you back to normal quake mode, useful for close combat nailgun battles. Use your regular grenades to protect yourself also (bind "+gren1" and "+gren2").
REMEMBER: tf has eliminated the "one shot, one kill" strategy. Hitting a player's head does double-damage, which usually frags. The legs do half-damage, and slows the enemy down a bit. All other body parts are normal damage, but if you hold the trigger on autozoom for like 3 seconds you can frag opponents most of the time.

AGAINST

Sniper hunting is a fun assignment in TF. Try and use a class with a rocket launcher (Pyro, Soldier) against Snipers. Try and learn where snipers hide in the level, usually dark, hidden crevices and ledges with a wide view of the level. Look for the bright red mark on their uniform to give them away. Also look out for the red crosshairs they project when they are aiming, that can tell you exactly where they are. When you find a Sniper, switch to your most powerful weapon and get ready to blast on him, usually they're too busy scoping the terrain to see you sneak up behind them.