Alternative or Alt. Rock is a very broad and rather vague term. Originally it encompassed almost any post-punk, non-mainstream music, from REM and the Pixies to XTC and Fugazi. In the early 90s, thanks to the success of Nirvana, Sonic Youth, and the Red Hot Chili Peppers (mostly Nirvana), the lines between Alternative and Pop were blurred. Pop bands adopted Alternative trappings and Alternative bands were marketed to Pop audiences. Everything from the alt-arena Rock of Soundgarden to the retro-pop of Smashmouth is classified as Alternative these days. Once your 14 year old little sister gets over N’Sync she will be totally into Alternative Rock.