Alternative Rock has of course meant a number of things over the years. In the 1980's it would be used to define a number of artists who offered an alternative to the commercial charts and would make good use of things like college radio and independent record labels to promote their music.
Today the term Alternative Rock is probably most widely used as an umbrella term for rock music that does not typically follow current trends or mainstream culture, regardless of the level of commercial success and rather than to describe a specific sound.
Wiki
RYM