what is the difference between unrated an the rated movies

Unrated films have generally not been through the country's rating process, so they haven't been given a rating.

For the US store, from http://en.wikipedia.org/wiki/Motion_Picture_Association_of_America_film_rating_s ystem :

If a film has not been submitted for rating or is an uncut version of a film that was submitted, the labels Not Rated or Unrated are often used. Uncut/extended versions of films that are labeled "Unrated" also contain warnings saying that the uncut version of the film contains content that differs from the theatrical release and may not be suitable for younger children or minors. When a movie is not submitted for rating, it means the MPAA is not sure what to rate it, and therefore may have content that is a mix of various ratings. Previews and trailers might also be approved for certain audiences only, but some may be approved for all audiences as well.