PSA Definition

A PSA (Public Service Announcement) usually is "advertising" a public or community event, generally for a non-profit organization. It is usually submitted electronically, through the radio, television, or another form of electronic broadcast. PSA's can also cover health and safety, and well as law and conduct.

Definition from: http://www.press-release-writing.com/how-to-write-public-service-announcements/

Examples: http://www.flu.gov/psa/