Cubs are the lowest ranked Garou, teenagers who have just changed for the first time, and have yet to undergo the Rite of Passage. (Or, 'been Rited.')
Before they change, they are, to all intents and purposes, human (and/or wolves), and don't ping as anything but that on Scent of the True Form.
Somewhere just around their first change, the cub's Kinfetch goes off and fetches the Garou, and the cub is brought into the fold.
If the Garou does not have a Kinfetch or a clear lineage, or if he did not change at the usual time, he is known as a Lost Cub.
On GarouMUSH, characters tend to be cubs for anywhere between three months and a year. It's strongly ICly encouraged for tribes to Rite their cubs between three and five months in, but events sometimes happen and the time as a cub gets extended.
They should be learning any number of specific things, as well as (in theory) embracing this attitude change. (For more details, see Cub knowledge)
Cubs are in the rather annoying position of not being adults, and are therefore not privileged to have adult roles in society. They may not join permanent packs, they may not hold Sept, Caern, or Moot positions, they may not be Guardians, and they may not Challenge adult Garou. Most annoyingly, they may not join Revels or go into combat situations unless given permission by their tribal elder (or tribemate teaching them most often, depending on the tribe) and have the okay of whoever is leading the combat party.
New players who are experienced with Werewolf and eager to be involved in plots may well wish to apply for Cliath characters. On the other hand, character development is rich and profitable, if you play a cub.