I'm not familiar with freelancer.com, but it sounds similar to contently.com.
As for taxes, if they pay you $600 or more in a year they would be required to issue you a 1099. They likely will request a Form W-9 from you prior to issuing payment. (at least, they should.) however, regardless of whether they issue you a 1099 or not, you are still required to report all earnings for income tax purposes.