From the Halo 2 Manual (Human Version): Not as readily understood as Elites, Brutes fight together in a pack and are physically stronger. Brutes demonstrate similar battlefield abilities to Elites, and their numbers have demonstrably increased since the conflict began. They carry a ballistic explosive weapon with an attached bayonet device.
From the Halo 2 Manual (Covenant Version): The newest ally to join the embrace of our flock is a powerful fighter indeed. The Jiralhanae [Brutes] are endowed with prodigious physical strength, and their simian origin makes them idea l for missions in any enviornment. We shall deploy them against humans in the same instances as the Sanghelli [Elites], but we may also give the Jiralhanae some increased security responsibilities in this conflict.
The Brute, known as Jiralhanae to the Covenant, is a completely new unit. Part bear, part ape, and all killing machine, the Brute makes a fearsome enemy. Although it lacks an energy shield like the Elites', it is a force to be reckoned with because of its sheer physical strength. Two of the game's weapons, the Brute Shot and the Brute Plasma are wielded exclusively by this race.