A douche bag is basically a douche — a product women use to clean their vaginas. But in slang it's an insult usually hurled at slimy, stupid men.
This is an extremely popular word. Many people seem to see douche bags everywhere they look. Despite how commonly douche bag is used, it is considered an inappropriate word in most settings. Parents and teachers certainly don't want to hear about how you think your brother is a douche bag. Be careful who you call a douche bag: in many cases, it will be safer to call them a jerk or idiot instead.