The term "freelancer" is used to refer to someone who works for him or her self, selling his or her services to others for short time periods. This is in contrast to someone who works directly for a company.
The most common example of this is a freelance journalist. Such a person looks for his or her own stories (or photographs), produces them, and then tries to sell them to a newspaper or a magazine. They are independent workers, not employees.
The term comes from the idea of a medieval knight who held loyalty to no one master. His lance (his main weapon) was said to be free because it (as a symbol of his service) was not committed to any one person but was free to work for whoever it wanted to.