"Alternative" encompasses a great many subgenres, and can cross over with various mainstream genres, and while it's often called "alternative rock", there are some subgenres of alternative that I wouldn't call "rock" at all. It has its origins probably in the 1970s, but "alternative" is a type of music that I'd say is most commonly associated with the 1990s, with the emergence of subgenres such as "grunge", "Britpop" and "indie rock". The genre is so broad that I won't always know whether an artist is considered alternative or not, so I'll pretty much defer to Wikipedia to decide whether or not to list them in this category. (Though I may sometimes disagree, even if an artist is called alternative, I may place them in a different category, such as "punk".)