I meant to add that today, aside from the military meaning of freelancers, the word describes anyone who offers his or her skills to a company as a contract employee rather than a regular employee, usually working for an hourly wage with no benefits or for a fixed amount on a temporary contract basis. Many people displaced by the recession beginning in 2008 work as freelancers because they cannot find permanent, full-time employment.
The origin of the word freelance comes from about the middle of the 1400s and described knights who no longer had feudal service obligations to a particular lord but who wanted to continue fighting for money. They were, in essence, the first professional mercenary military troops.
It's quite likely that the term and concept of freelance were actually a by-product of the Crusades. Between about 1100 CE and 1300 BCE, tens of thousands of knights followed their leaders from Europe to what is now the Middle East (specifically, the area of Israel and the city of Jerusalem) in order to conquer these areas and Jerusalem and return them to Christian rule. After engaging in a number of Crusades, Muslims finally defeated the last group of Christian crusaders, and the survivors of these struggles made their way back to Europe where the feudal system was beginning to break down.
There were now thousands of knights who had no particular lord to give their allegiance to, so they offered their services to anyone who needed a well-armed, well-trained cavalry soldier. The more experienced and renowned a person was, the more he commanded in terms of salary, and these men came to be called freelance knights or freelancers because they owed no allegiance to anyone, and their services could be bought by anyone who could afford them.
With the advent of the freelancers, much of the status of knighthood began to disappear because the freelancers were no longer fighting for someone out of obligation or for a belief. There were, of course, many knights who still had feudal obligations and fought for a belief system, but they were vastly outnumbered by freelance knights. Although the original freelancers were, for the most part, knights and cavalry troops, over time any soldier who sold his services came to be described as a freelancer.
Today, mercenary soldiers are the descendants of the original freelancers.