Contrary to the talk, it is a reasonable, limited, carefully-crafted measure designed to help law enforcement deal with a serious problem in Arizona. Its authors anticipated criticism and went to great lengths to make sure it is constitutional and will hold up in court. It is the criticism of the law that is over the top, not the law itself.
The law requires police to check with federal authorities on a person's immigration status, if officers have stopped that person for some legitimate reason and come to suspect that he or she might be in the U.S. illegally. The heart of the law is this provision: "For any lawful contact made by a law enforcement official or a law enforcement agency…where reasonable suspicion exists that the person is an alien who is unlawfully present in the United States, a reasonable attempt shall be made, when practicable, to determine the immigration status of the person…"