Peer support is a supportive relationship between people who have a lived experience in common. The peer support worker provides emotional and social support to others who share a common experience.
(Mental Health Commission of Canada: http://www.mentalhealthcommission.ca/English/focus-areas/peer-support)
Peer support can be offered informally or casually through social interactions or recreational activities, but it is often organized and run as a structured, formal program by persons with lived experience who are trained specifically to provide support.