An Antitribu, or antitribe, is a vampire who has rejected the political allegiance and/or philosophical leanings of its parent clan, and either gone independent or joined an opposing faction. All members of the Anarchs, for example, are effectively antitribu, and both the Sabbat and Camarilla feature many members whose original clan are officially part of the opposing organization.
For many antitribu, the philosophical differences between themselves and their parent clan may be small, or only a matter of degree. For example, most Brujah Antitribu among either the Anarchs or the Sabbat basically take the Brujah fondness for chaos and anarchy and turn it up to eleven, rejecting the restrictions imposed upon them by the codes of the Camarilla and striking out on their own in search of greater freedom of action to do as they please. For others, it is purely a political choice: very few Nosferatu reject their parent clan, as its the only place such monsters as them can find anything close to real acceptance, but a surprising number have rejected the Camarilla and found something of a home among the Anarchs or Sabbat instead.
For many other antitribu, however, the rejection of their parent clan, its parent umbrella organization, and everything they stand for is a profoundly important part of their own self-image, and they revel in violating the most sacred precepts of their parent clan. As most vampire clans are fairly nasty groups in the best of times, this means that Antitribu include both some of the nicest and most moral vampires out there...and some of the most twisted monsters in existence, who reject even the few scraps of moral behavior adhered to by most other vampires out of sheer self-preservation if nothing else.
Antitribu generally have many or most of the same powers and abilities as others of their parent clan, but very different habits or predilections. As many of them announced their split with their parent clan by diablerizing their own sire, they often have also diablerized other vampires and taken their powers, so it's also not unusual to find antitribu with bizarre combinations of powers and abilities stolen from many clans other than their own original family.