What's the Difference between a PSA and a Teaser Trailer

PSA is short for  Public Service Announcement. A PSA is a noncommercial advertisement on radio television or film to raise attention and awareness on a given topic i.e.;Smoking , Aids The Crisis in Dafur.

A Teaser Trailer is and advertisement of a film or TV Show. It is used to promote a film or show.

The reason the word "teaser" is because it is just that a tease to gain the interest of the audience  it has very little or no clips of the film just subtle hints of the film or show, and ends with a reveal of the Main title and release date.

and finally here are two more examples of a PSA and a Trailer