A Framer may choose to frame a player during the night, causing that target to investigate as Guilty (or whichever result is unfavorable) to Cops. For obvious reasons, this role is almost always aligned with scum.
No version of this role is currently considered Normal on mafiascum.net; Mafia Framers were allowed prior to the spring of 2013.
The most common variant of Framer is Tailor, which can also target scum to have them investigate favorably to Cops. The name comes from the role tailoring investigative results to the user's liking.
The reverse, a role that gives incorrect innocent results, is the Lawyer.
In games without Cops, Framer/Tailor variants can exist that alter results given to other investigative roles (such as Tracker).
Use and Power
Framer exists solely to mess with Cops, and is a means of tempering Cops' results. However, most of the time Cop investigations will not be broadcasted for the Framer to directly tamper with, so in practice the Framer must try to deduce who the Cop will investigate assuming that the Cop is not the person the Mafia is killing at Night. This is not impossible to do, but at the same time not particularly likely, so Framer is roughly as powerful as Godfather.
Tailor is slightly more powerful, as it prevents Cops from believing their innocent results as well. Much of the effect of these roles comes in causing players to doubt the Cop's results after they flip (and thus the Town becomes aware of the possibility of the investigation results being tampered with).