What is a cam girl and what do they do?
A cam girl is a paid performer, usually adult-oriented but sometimes non-nude, that entertains viewers on the Internet via a webcam. While there are cam girls who host their own "solo site", it is very popular to work on an existing cam site as a cam girl.
A cam site takes care of the technical work (setting up, programming, and hosting the site as well as providing an intuitive platform for the cam girl to use), the advertising, etc., so that the cam girl only has to focus on the actual cam shows. Normally a cam girl on a cam site is an "independent contractor" for the cam site, not an employee. She purchases her own equipment (web cam, computer, Internet service, lighting, props, etc.), makes her own hours and decides what type of shows she'll do. Some girls may choose to work for a studio, and sometimes studios provide the previous items for the girl, depending on the contract. See Studio vs. Independent to learn more about what studios may provide as well as pros and cons.
This wiki is more tuned toward independent cam girls who work on existing cam sites.
What do they do?
That's really up to them and the rules set forth on the cam site she works on. Many cam girls' shows are sexual in nature, getting nude and using toys or fingers for masturbation. Some are very teasing and alluring and use body language (as well as spoken language) to titillate viewers. Some cam girls display their extraordinary talents, such as hula hooping, playing an instrument or singing, gymnastics or even juggling to entertain their viewers. There are even cam girls who simply chat with their viewers in such a way that friendships are formed.
Different cam sites allow or disallow certain acts or types of shows, or restrict certain acts to paid shows. See Types of shows for more details about which show types you may expect on a cam site.