Grunge style clothing originates from the late 1980's to the early 1990's when grunge music was at its peak. Bands such as Nirvana, Alice in Chains, Soundgarden and Pearl Jam were the most popular of this new-found genre of punk music that created a social movement for the youth during that period.
The grunge style fashion look appears to me as though the trend originates from punk. Torn jeans and ripped shirts, topped off with some flannel is a typical grunge style look.
Grunge music is a mix of different musical genres such as punk, indie and heavy metal, which to me is why grunge style clothing differs from the aforementioned other genres.
Like most of the things that we wear, I believe it is a statement about what we stand for as well as how we are feeling. Grunge is about attitude and rebellion. It sits far, far away from the squeaking clean pop look, and is about being comfortable with yourself and not caring what others care about you.