Plan B works by providing a high dose the same hormone that makes birth control pills work, thus preventing pregnancy when taken within a 72-hour time frame (its less effective the longer you wait). It's not meant to be a method of birth control - it's meant for emergencies only. Plan B One-step is available over-the-counter to women 17 years old and up.
This great piece by Kelly Conlin in the Huffington Post ("Women Need a Plan B") details the quiet importance of this moment in contraceptive history - the improvement and approval of a contraceptive without someone on the far right throwing an ill-informed temper tantrum - and frankly it speaks of a refreshing embrace of common sense that many of us really need to hear right now. In the disgraceful circus the sore losers on the right have made out of health care reform the simple, sensible maturity of women quietly opting to get the care they need to prevent unwanted pregnancy is the best single dose of news I've heard in quite some time.