Growl is a hunter pet ability available to all pets, that allows them to taunt their targets and keep the enemy off the hunter.
Growl is set to autocast by default, which is often the case of annoyance in dungeons, as a growling pet will steal threat from the tank (potentially resulting in overstressing the healer, or the pet's death). In solo combat, however, Growl is usually best kept on so the pet can tank the mobs and keep the hunter free to shoot.
In short: turn Growl off when entering a dungeon; turn it back on when playing solo.
Patch 6.0.2 (2014-10-14): Now has a 30 yard range (up from melee).
Patch 5.0.4 (2012-08-28): Growl is now a taunt, instead of just increasing threat amount. Cooldown increased from 5 to 8 seconds.
Patch 1.12.0 (2006-08-22): Now correctly initiates combat when used by a pet in passive mode.
Patch 1.7.0 (2005-09-13): To help make "hunting" pets more viable in more situations, the Growl ability is now free to all pets.