Limpy is a standard image based captcha. It currently picks 4 letters, numbers or symbols at random and applies a random font to each character. These characters are then rotated and placed on top of a random background.
An image is then generated and shown to the user who has to figure out what these 4 characters are. Easy right? Well, maybe not. Sometimes these are unsolvable for people with good eye sight, never mind users with visual impairments!
This example is here to demonstrate this form of Captcha if you haven't come across it before.