Camping
From Halopedia, the Halo wiki
Template:Realworld Camping is the act of voluntarily keeping out of firefights by staying in one place while shooting from a distance or sneaking out from behind a corner to wreak havoc on an unsuspecting player passing by. Camping is rarely performed unless one has a powerful weapon such as the Sniper Rifle, the Rocket Launcher, or the Energy Sword. Although camping is a legal and legitimate strategy, most players complain that it takes away from the inherent fun of the game.
Camping is also used in areas where an enemy needs to go (bomb/flag) or cannot stop moving (i.e. lift). The player sits in a hard to see place or at least a place to get a jump on the enemy and kills them quickly.
A variant of camping known as "Spawn Camping" involves exploiting spawn points by camping near them and catching recently spawned players offguard and poorly-armed.
Another type of spawn camping is to drive a vehicle (e.g. warthog, or ghost) around your enemy's base. That is, even though players spawn at random places, with enough players you'll be able run over a lot as they spawn around their bases. Or a person could just park a tank near the base and obliterate anybody who spawns there.
Although there are many ways to camp, one of the most common is to camp the purple lifts of the Halo 3 Multiplayer Map Construct with a sword, catching the unexpected people going up the lift when they can not stop moving, and have no way of fighting back. Some players will attempt to avoid having this happen to them by sending a grenade up the lift before they ascend themselves, clearing the way.
Once a camper's location has been determined however, it is generally very easy to take him out. A good way to take him down is to pitch a grenade around the corner to flush him out, then finish him off when he leaves cover (or just keep pitching grenades until he dies).