These jobs are marketplaces where you can market yourself and place bids on projects that companies have posted. This is great for people who would like to control how much, or how little, they work. To do this line of work, you have to be organized and responsible so that you don't miss any deadlines.
Sign up is free on all of the sites. However, some sites will make freelancers pay for extra bids. For example, you may get 10 or 20 free bids a month on a site, but will have to pay so much a month to get unlimited bids. It's nice that they allow you to start out free so that you can decide whether or not you even need unlimited bids.
With marketplace jobs like Elance, Get a Freelancer, and oDesk you can make money for your skills by bidding on projects.
Elance: Elance allows you to market your skills and find companies that post jobs they need to have done. Writing, art, graphic design, editing, there are all sorts of jobs you can find on Elance.
Get A Freelancer: Market your skills and find companies that will hire you to contract work for them!
oDesk: oDesk is similiar to the other sites because you market your skills and find others who will contract you to do the work.
Virtual Office Temps: Virtual Office Temps connects workers to companies. You build a profile and market your skills. Companies look for people to do data entry, clerical, legal, and medical office work.