Though the genre is considered to be rock, some of its subgenres are influenced by folk music, reggae, electronic music and jazz among other genres. At times alternative rock has been used as a catch-all phrase for rock music from underground artists in the 1980s, all music descended from punk rock (including punk itself, New Wave, and post-punk), and, ironically, for rock music in general in the 1990s and 2000s.
While a few artists like R.E.M. and The Cure achieved commercial success and mainstream critical recognition, many alternative rock artists during the 1980s were cult acts that recorded on independent labels and received their exposure through college radio airplay and word-of-mouth. With the breakthrough of Nirvana and the popularity of the grunge and Britpop movements in the early 1990s, alternative rock entered the musical mainstream and many alternative bands became commercially successful.
http://en.wikipedia.org/wiki/Alternative_rock