What exactly is a "Hail Mary"?

The practice of a saying a "Hail Mary" is often referred to in colloquial speech, sometimes as a way to caricaturize Catholics. References to the practice instantly conjure up images of Catholicism, priests, and confessionals. However, beyond these references with my Protestant background I don't actually know much about the practice. I don't even have the vocabulary to properly ask this question. What should a non-Catholic like myself understand about the practice? Where did it originate and what is the purpose? Do other traditions employ such a ritual? Is it fair to call them a ritual? **Edit:** If it's simply a prayer, what is the purpose of repeating it more than once? At least the pop-culture references often involve saying "so-many" Hail Mary's in the sense of more being somehow better.