What are green day songs all about?

Obviously this depends on the song in question, but many green day songs are a comment on the problems with modern American society, mainly drugs and the hypocrisy of politicians like George W Bush. Examples of these would be American Idiot and Minority. Others, like Wake Me Up When September Ends and Good Riddance (Time of Your Life) are about past experiences such as bereavement and relationships respectively. Another theme is war (see Holiday) I would suggest looking at the lyrics of each song you are curious about for a fuller understanding of its meaning.