A PSA ("Public Service Announcement") is a "message in the public interest disseminated by the media without charge." In other words, it's an advertisement with information for the general public, run for free on TV, radio, billboards or magazines.
What the definition doesn't say is that a PSA has to be:
Do you recognize any of these images?
How about this recent radio PSA from the CDC -
Tips for creating a great Public Service Announcement ---
- Know your GOAL - to get people to take a specific action (stop littering, wash hands, stop texting while driving)
- Talk about RESULTS - a beautiful countryside, not getting bird flu, not causing a car crash)
- Use informal EVERYDAY LANGUAGE (not lit writing or police speak)
- Use EMOTION
- Make it PERSONAL
- Stick to ONE MESSAGE
- Remember that SOUNDS CREATE PICTURES. If you use sound effects or music, it must make the right picture in the listener or viewer's mind.
- Keep it SHORT. 30 seconds is very common.
- IDENTIFY the sponsoring organization.
How to Create the Perfect Public Service Announcement (Center for Digital Education)
How to Write a Public Service Announcement (pdf, from Kansas Association of Broadcasters)
Public Service Announcements (Wikipedia article)
Distracted Driving video PSAs (The Hill)
Sound Effects clips
Sound Bible (wav & mp3 files)
FreeSound (wav files)
Big Sound Bank (wav, aiff, mp3s; a French site)