The common name used for the US State Department's Diversity Immigrant Visa Program.
Each year the Bureau of Consular Affairs of the United States Department of State organizes a lottery in which people meeting certain requirements can win a permanent residence visa. The reason for this is to increase the diversity of the US population, and is weighted between geographical regions and countries, so that more visas goes to regions with lower immigration rates.
In the lottery that takes place in 2001, known shortly as DV-2003, no visas will be going to citizens of countries that have sent more than 50000 immigrants in the last 5 years. This means that if you are born in Canada, mainland China, Colombia, Dominican Republic, El Salvador, Haiti, India, Jamaica, Mexico, Pakistan, Philippines, South Korea, United Kingdom (except Northern Ireland) or Vietnam you will automatically be disqualified. Otherwise you do have a chance.
In spite of what some people say there is no special form to fill out, neither a fee. All you have to do is follow the fairly simple instructions found on the State Department's web site (see below), and make sure that the application is sent between to the correct address between 1 and 31 October 2001. No need to use the services of shady green card lawyers or other middle men.
For more information, see Instructions for the 2003 Diversity Immigrant Visa Program (DV-2003) on http://travel.state.gov/visainstructions.html