Green Day is a pop-punk band from northern California.
The name itself has caused much trouble for the band. They were thought to be environmental extremists and placed alongside The Insane Clown Posse and Debbie Gibson on the US government list of terrorist groups. On the other hand, they had been invited to be on Happy Tree Friends and VeggieTales.
They are probably more pop than punk, with even Johnny Lydon saying that they did not have the proper punk credentials despite being DIY types. However, they make accessible music versus the original punk bands that generally lasted all of a year. Still, that popularity led to others creating hundred of bands that sounded exactly like Green Day, using the same three beats and three chords.