Trailer vs. Teaser?

My dictionary says a trailer is

a short motion-picture film made up of snatches from a feature picture and displayed in advance for advertising purposes

while a teaser is

an advertisement meant to arouse curiosity sometimes by withholding part of the material information

So a trailer is a complete work that uses excerpts to pique your interest, while a teaser uses just enough of a work to get you interested in seeing or hearing more.

A teaser better describes the music previews that iTunes and other online music stores provide.

Teasers are usually shorter than full trailers. Teasers typically come out before the full-length trailer to get fans excited before much of the movie footage is available.

For music tracks, I've typically heard partial parts of the songs as samples. Full length tracks released before the album comes out are typically called advance copies or pre-release tracks.