In terms of taxes, what forms do you need to submit as a freelancer?

[danalana]

As a freelancer, are you considered to be self employed? If so, which forms do you have submit and fill out? What does it mean to have to pay quarterly taxes as a freelancer? Is there an amount that you have to reach in order to need to pay taxes? For example, if you only earn 50 dollars this year, how much taxes do you have to pay and how do you calculate the tax liability?