Within clubbing, rave, and other party cultures, 'Visuals' are a form of visual performance art. Most often these artists are called VJs. (Short for video jockeys) Often relying on computer generated, or procedural artworks, but they are not limited to this, as many artists incorporate other things like film, photography, video feedback, and illustrated animations.
Most often these 'visuals' are projected behind a DJ or band during musical performances to augment the experience. Although they are commonly set up to work on club monitors.
This is most often distinct from intelligent lighting, or laser light shows, although some artists might also incorporate this into their style.
Sometimes these visuals are pre-rendered or prepared in other ways before the event, other times music responsive programs are used that respond to the audio happening live. Other times the VJ might change elements of the visuals in other ways to line up with the musical performance.
I went to the club last night, Bad Boy Bill's set had some crazy visuals created for him specifically that really blew a lot of people's minds.