It's a crappy, false, destructive set of meme complexes that's largely propagated through inheritance/indoctrination of children, and sometimes, or at least historically, through war/conquering.
It's an appallingly black-and-white way of thinking about the world and the universe, and it projects archaic human notions of morality, ethics and values onto God, elevating them to absolutely righteous and immutable status.
It stunts people's emotional growth and makes them suffer through the idea that God judges us and through guilt, which is never helpful.
Religion is metaphysics, mysticism or spirituality filtered through the eyes of the masses and psychosocial interaction on a societal scale. It's a lowered and degraded distortion of true enlightenment, where that distortion ranges from moderate to severe depending on the religion.
Unlike most atheists, though (not that I'm an atheist), I don't view all religions as being equally nonsense. Some religions are definitely better and truer than others. Buddhism and Taoism, for example, are much better than Christianity and Islam.