Elance, Odesk and other sites like these ones are great for specific project(s) like having a website built or video editing.

You can expect to pay higher hourly rates due to the fact that it's short term, freelance type work.

Typically, you'll also find lots of companies and firms that have built themselves a reputation and a presence on these sites. They'll be bidding to do the work instead of a single freelancer - again because it's project based and lots of web design agencies can do the work pretty quickly.