In Vampire: The Masquerade, the majority of vampires are loyal to one of several sects, which are strongly clan-oriented but primarily political entities. The two major sects are the Camarilla and Sabbat, which respectively comprise vampires who attempt to uphold the Masquerade and generally maintain their humanity at passable levels, and vampires who seek to eliminate the Masquerade and rule humanity as predators. This isn't to say that the Camarilla are nice, they're just pragmatic.
These sects trace their history back to 16th century Europe and the Convention of Thorns which ended the Anarch Revolt. The Sabbat are the descendants of those Anarchs, while the Camarilla are the signatories to the convention. The two sects have been mortal enemies ever since, although the difference is blurred at the higher levels and lower generations.
In addition to these two sects are two vastly older sects. The first, the Inconnu, are loose affiliation of vampires who have had a common identity since approximately the fall of Rome. The Inconnu generally avoid all vampiric politics, and may only be a sect by virtue of being too powerful and collegial (with each other) to be considered independent. The second, and far older, is the Black Hand: a Gehenna Cult which traces its origins to the pre-Christian Middle East. The Black Hand is now a subsect of the Sabbat, but its members have their own agenda of which the Sabbat is a part.
In addition to these divisions are some known sect-less divisions. The most well known are the Independent Clans: Assamite, Follower of Set, Giovanni and Ravnos. These clans have their own internal structure (which are fairly sophisticated for some of the clans) and generally have no truck with any other organization. The Anarchs, who in modern night are a loose (and largely extinct due to the New Promise Mandarinate) confederation of vampires generally adopting a combination of youthful rebellion and less dusty political thought. Finally, there are vampires termed Autarkis, meaning that they exist independent of the entire structure - generally ruling small communities absolutely or otherwise hiding from all support structures.