The rave is a meeting of the Brujah clan. Rants are large and (relatively) structured affairs that are often preceded by a smaller, more informal rant.
Raves are formal meetings of the Brujah, often including members of different clans and even a few mortals. These are huge social gatherings often masked by entertainment events such as concerts, protest rallies, or anarchist group meetings to bash "the Man". These events are well organized and are as formal as the clan of Rabble ever get. Speakers all get a turn to speak, unlike a Rant where they shout a point while many others are shouting. Clues to the next Rant and Rave can be found here as well.
References[]
VTM: Clanbook: Brujah, p. 27-28