Beast Masters (Commonly abbreviated as BM) are Rangers who, instead of focusing on dealing damage through their bows, focus on strengthening their Pet, and use almost exclusively pet-oriented abilities. They were previously almost non-existent, but now, thanks to the numerous improvements, through both Updates and the addition of Factions, Nightfall, and Eye of the North, they are slowly gaining acceptance as a plausible character type within most communities.
Beast Masters are a cross between a Minion Master and a Warrior. They resemble Minion Masters because of the fact that they bring another ally to the group, without taking up a slot (at the price of being only effected by abilities that affect 'allies') though resemble Warriors specifically because of the fact that depending on the build the Beast Master uses, they can either take an enormous amount of damage, through the use of Defensive Shouts, or utilize many Pet Attack skills that are made to dish out large amounts of damage, which rivals, or even exceed at times, the offensive capability of even some very high damage Elementalist abilities. Or, at times, a Beast Master takes a role that falls in-between a pure attacking build and a defensive build.
Some common Shouts (Or Skill, in the case of Feral Aggression) used in a Beast Master skills include:
Feral Aggression, and Call of Haste are most often used in a role where they want the pet to be dishing out large amounts of damage, or pursuing targets very quickly as well, in the case of Call of Haste.
Otyugh's Cry is much less common than Call of Haste or Call of Protection, due to its history as being referred to as the worst ability in Guild Wars, but ever since having its functionality changed in the 2008/4/10 Update to give your pet +24 armor and not able to be blocked, it is gaining acceptance within the Beast Mastery community.
Pet Attacks that are used in many Beast Mastery builds are:
Enraged Lunge is one of the most common, partially because it has the largest damage bonus of any Pet Attack, and in PvP, at level 13 Beast Mastery or higher, it requires only 3 other Beast Mastery abilities to include the full damage bonus of 80.
Brutal Strike is generally used when a Beast Master does not wish to use Enraged Lunge as their elite ability.