actually, alternative is a sub-genre of rock, itz actually alternative rock....and as you probably know there are many genres of rock( i.e. punk rock, progressive rock, glam rock, etc.) the thing is bands tend to get classified into genres rather then the music(song) in question, which is why heavey songs are classified as alternative and vice-versa. what makes it difficult to classify alternative really is that alternative rock doesnt really have the distinctiveness of other genres like glam rock or death metal, it has always been more of a generalization of anything which is not mainstream.