Book Trailers are similar to a movie trailer, in that they are designed to build interest in an upcoming or current novel and to encourage people to buy the book that they are based on.
The main difference is that a movie trailer already has visual images to work with - clips from the film. With a book trailer, the maker (either the author themselves or a professional videographer) has to convert the written words into visual images. The trick is to convey a sense of what the book is about without giving anything away - and without really clearly defining what the characters look like, as most readers prefer to visualize what they are reading about as they imagine it themselves.
Most book trailers run from one to three minutes. They can be anything from the author reading a passage from the book, to an elaborate mini-movie.