A typical greeting done by a female, it has long been considered to be a gesture of respect, though it is less common nowadays. For all variants of the curtsy the female bends her knees, sometimes sweeping one foot behind the other, and holding her skirt in both hands. Can also be done in a mocking manner, as a sign of disrespect. It's also a common part of those Regency-Debut to London Society type books. Specifically, the formal presentation to the queen, where the presentee has to curtsy before the queen. There's a little info about it here, after the fourth picture.