http://www.usafis.org/What is the U.S. Green Card Lottery?
Each year, the Diversity Lottery (DV) Program makes 50,000 immigrant visas available through a lottery. If you receive a visa through the Diversity Visa Lottery Program you and your family will be authorized to live and work permanently in the United States.
Important to all visitorsGreen Card Lottery Application, will be available only online at the State Department dedicated website - No more regular mailing of applications.
From the State Department Rule for the DV Program:
"Alien petitioners for the Diversity Visa Program will no longer be permitted to submit a petition by mail. Instead, the Department will require that all petitions be submitted to it in an electronic format, using an Internet website dedicated specifically to the submission and receipt of Diversity Visa."
Click here to read more about the changes made to this year's American Green Card Lottery Program.
Official U.S. Government Program
The Green Card Lottery program was established under the Immigration and Nationality Act and was approved by the U.S. Congress
More at link