Guilds are you way to powers in Ravenblack City. If you stumble across a Guild and have enough cash on you at that point in time, you can either buy a power right away or get information on how to fully quest for one, depending on which power you attempt to go for.
There are six Guilds in the city, all with a different theme, and all with different powers. This thread is to explain what power you can get at what Guild, and how it fits to the theme.
Guilds move on a regular basis. They tend to change the day following a day numbered one greater than a multiple of four (1, 5, 9,...). Sometimes this 'rule' is off a bit, so that generally speaking it always (in the past, at least) has held true that Guilds move every 3 to 5 days.