Flash mentoring is a way to informally connect with mentors. The relationship has no required relationship length, no formal "matching" and is primarily two individuals communicating to accomplish a simple goal.
Formal mentoring requires the filling out of a matching application, requesting to work with a mentor and having the mentor accept the relationship request. Sometimes program administrators control the matching of a mentor and mentee.
Mentees and mentors can be in both a flash mentorship AND a formal mentoring relationship.