The Bureau has an insanely long name. That's why it is mostly known as The Bureau. When you work for The Bureau, you bascially try to maintain some order in the Multiverse. The Bureau sort of monitors portal abuse. After all, who wants an army of clueless accidentally enter the City of Doors? The Bureau is rumoured to employ Seers, so that they can send agents into worlds before a portal abuse can happen. This is not entirely true, though The Bureau employs the odd Oracle. Overall, some agents do preventative work, some improve or study portals, some do 'aftercare' after some more or less catastrophic incident has happened. Needless to say, the Bureau is always busy!
The current leader of The Bureau is an aasimar called Amun (lawful neutral, figher/psion/wizard/cleric), a lover of the famous Tarsheva Longreach. Many people believe that Tarsheva also has some ties with The Bureau. Amun is actually more than 'just' an aasimar. He is a half-god. He is pretty much an atheist, though, but can still use his mother's spells (if he choses to, that is). The Bureau is also rumoured to work closely with the Sensates who are, in turn, rumoured to be Sigil's secret service.
The Bureau has its headquarters in a very unusual location. In what looks like a derelict building at the edge of the Hive. The Bureau is not actually located in the building itself, but in a sort of pocket dimension that can be accessed through a portal in the building in question.