There is a pub called the Wessex Arms, which all the characters go to on Friday nights - the Interactives are what takes place there.
There are many different Guilds in Grantabrugge, which many characters are members of.
There is a large College of Magic, which trains mages. There are 6 elemental flavours of magic, plus Grey, which is a combination of them all.
There are Churches to each of the gods, and priests of all 13. The Gods are active and directly involved in the world through their priests. It is possible for Gods to die, merge or schism. Until a few years ago there were lots more gods, but many died in 1286.
The everyday world is found on "The Prime". The 'known world' on the Prime for an inhabitant of Grantabrugge is basically an area similar to Europe and the top edge of Africa. "Above" the Prime are the 6 elemental planes (where magic comes from, ruled by Elemental Lords and inhabited by elementals) and the god planes (one per god, the plane actually is the god). There is also the plane of Arcadia, home of the Fae, connected to the Prime by the Dreaming. "Below" the Prime are lots of other planes, most of which are inhabited by and/or composed of demons of various sorts. Travel between planes is possible by ritual magic, usually to and from the demon planes, although it is not always straightforward. Many creatures native to other planes also know how to travel to and from the Prime.
Linears (held on Saturdays) usually involve a group of characters from the Wessex Arms travelling to another place (anywhere from a village just outside the city walls to another plane entirely) in order to carry out a mission of some sort, usually returning in time to get back to the pub by the Interactive next Friday.
This wiki also contains a Library of various in-character documents for your amusement and edification - you do not need to have read any of these documents in order to play, although several of them do usefully describe events in the history of the TT world (often in a biased way).