murder mystery games cast each attendee of an event as a suspect in a murder. Often based on "locked room" mysteries (think Agatha christie), they can span the gamut of serious to comical. 

Characters each have their own info packet, which fills in the information they know, secrets they're keeping, and might give direction about when that information should be revealed. Lots of variety in character booklets but these are the basics. 

The object of the game is to find the murderer. Many (not all) murder mysteries have mechanics to facilitate accusing/revealing the murderer, or conditions that, when met, force the murderer to reveal themself

Helpful Links:

It's a Me, Murder by Kristina Marina (the game is free)

Playthrough of It's a Me, Murder by the Moonshot Network

Wikipedia definition of murder mystery games

Fun List of other free murder mystery games