Both a PvE and a PvP talent. The pet of a Beast Mastery hunter typically deals between 30 and 40% of the hunter's damage. Over time, this amounts to huge numbers. Full Beast Mastery hunters will generally take Bestial Wrath in order to further increase the damage potential of their pet.
Patch 3.0.8 (2009-01-20): This talent now grants only 3/6/9/12/15% pet damage.
Patch 1.5.0 (2005-06-07): Fixed a bug where the damage increase was lower than stated in the tooltip. It should now increase damage by the listed amount.