Frenzy is what happens when the Beast within takes over completely.
Any number of things can cause frenzy: Embarrassment or humiliation; strong emotions such as lust, rage, or envy; extreme hunger; confinement or helplessness; being taunted by an enemy; the threat of silver in the area; being wounded or seeing a packmate wounded; and more.
Mechanically, to enter frenzy requires 4 successes on a Rage roll. (For more mechanical details than you can shake a stick at, please see 'news frenzy check'.) The player may spend a Willpower point right then to halt the frenzy, but her character loses any further actions that turn. (If the player gets 6 or more successes on a Rage roll, the character enters the Thrall of the Wyrm-- please see 'news thrall' for more details.)
Obviously, this means that technically, those with Rage below 4 cannot frenzy. Please use both common sense and sense of drama, here -- even the lowest Rage Garou /can/ frenzy, and all characters have triggers which can cause frenzy. And it will not make sense, in all situations, to use Willpower to stave off frenzy.
Non-mechanically, upon such events, the Garou's the fight-or-flight reflex goes into overdrive, and the Garou goes into one of two types of frenzy: Berserk, or Fox.
In a Berserk frenzy, the werewolf becomes a ravening beast, intent on attacking everything and anything that has anything to do with the situation that caused the frenzy. They see only red and moving shapes. They wish only to reduce these shapes to mangled carcasses. A berserk Garou shifts immediately to Crinos or Hispo (player's choice) and attacks.
Generally, the berserk Garou will attack anything that moves, but most usually she will attack the cause of the frenzy. Coming out of frenzy requires that the situation that triggered it be over. Generally, once there are no obvious threats left, the Garou will collapse unconscious, briefly.
The attacking Garou can (and indeed is required to by the mechanics) use Rage as much Rage as possible per turn, but they cannot use pack tactics, Gifts, cannot step sideways, and so on. They just attack. (Though, since they're so focused on attacking, they also don't feel pain, so wound penalties are decreased.)
If a Garou's permanent Gnosis exceeds her permanent Rage, she will not necessarily attack her packmates, unless they are the proximate cause of the frenzy or have attacked her in the interim. But even so, if the packmates are the last ones in the area... beware. The frenzying player may attempt to spend a Willpower point in order to avoid attacking their packmate, but it is by no means guaranteed to work. (If this succeeds, generally they'll fall unconscious.)
In addition, the Garou, under those conditions, will remember the frenzy. On the other hand, If the Garou's permanent Rage exceeds their permanent Gnosis, the Garou will not remember the events of the frenzy.
A character who is Fox frenzying is fleeing in terror for his life. He shifts to Lupus form and runs, attacking anything that gets in his way (although more with the intention of getting past than of killing). Once the character reaches a safe hiding place, he will remain there until the frenzy passes.