There are normally two different types of trailers; a teaser trailer and a main trailer.
A teaser trailer very quickly advertises an upcoming film, a while before it actually comes out. These can be found on television and at cinemas. Usually, the teaser trailer is between 30 and 60 seconds long, with minimal footage to literally tease the audience. Due to such a short time for everything to be shown in the trailer, the pace is rapid and scenes can be cut right in the middle to go to the next one, leaving the audience wanting to know more. A simple yet effective method of advertising a film.
The main trailer usually lasts a bit longer, and is released closer to the film's release date. The trailer shows a lot more of the film itself, and usually shows the best bits. This can sometimes be known as a 'spoiler', if a trailer shows particularly golden bits of the film. However, to promote a film, the best bits must be shown. Therefore nearly spoiling a film is unavoidable. On the other hand, horror trailers are usually a lot less informative, occasionally just showing a lot of dark scenes, or as with 'The Blair Witch Project', just actors running with no information given away at all.
Music in a trailer can vary depending on the genre. This could actually be said for all conventions of a trailer; it depends on the genre of the film how the trailer looks.