Alternative rock its a genre of rock that consists in all that rock music than not get into another genres like: punk, grunge, heavy metal,etc. Personally i considere it more musical than that heavy and loud punk or grunge music, it expresses feelings and is more deep that any other genre, besides it sounds great, and it isn`t so emotional hardcore.
Some of the more famous groups of this genre are:
Goo goo dolls
This last one is my favorite with its song, iris that for me its really cute