The symbol for Slayer in Halo.
Slayer is the basic game type throughout all the Halo games. It is similar to what is known as "Deathmatch" in many other games. Although there are many variants of Slayer, such as Team Slayer and Free-for-All, the basic idea is always the same, whoever reaches the kill limit first, wins. It is by far the most played game type.
There are a number of slayer variants in the Halo: Reach Multiplayer Beta, while more variants are expected in the game's final release of the game. Below are a few of the major types of Slayer.
 Default Slayer
The default version of Slayer is the most basic. It is a free-for-all match with the score limit of 25 kills to win. The starting weapon is the Assault Rifle, there is no secondary weapon. The shields, damage resistance, etc, are all set to their normal settings.
 Team Slayer
Team Slayer is a very common Slayer variant. Its basically the same as default Slayer, except that there are teams involved instead of it being a free-for-all match. The score limit is of 50 kills to win.
 Team Swat
Team Swat is another common Slayer variant. It is the same as the default Team Slayer, except it has a DMR, or the Designated Marksman Rifle as the startng weapon, and no given secondary weapon. In match-making, the score limit is of 50 kills to win.
 MLG Team Slayer
The MLG variant is another common Slayer variant. MLG stands for Major League Gaming. It is the variant used in the MLG Pro Circuit tournaments. There are a few differences between this and normal Team Slayer. First of all, the starting weapon is switched from the Assault Rifle to the Battle Rifle. Also, movement speed and the damage modifier are increased slightly. This allows for a more intense experience and requires more skill. The MLG Team Slayer variant is often combined with a special MLG map variant created in Forge. This allows for people to have the complete MLG competitive gaming experience.
 Team Snipers
Team Snipers is the same as Team Slayer except for the weapon setup. The starting weapon is the human Sniper Rifle. The secondary weapon is the Magnum. You also have infinite ammo and there are no weapon spawns on the map.
 Shotty Snipers
Shotty Snipers is a common Slayer variant that comes up quite a bit in matchmaking. Its a Team Slayer game with the starting weapon being the Sniper Rifle and the secondary weapon being the Shotgun. Just like Team Snipers, you have infinite ammo and there are no weapons spawning on the map.