From Irish Emigrant: good summary:
The Diversity Immigrant Visa Program is administered annually by the State Department and to give up to 55,000 visas to persons from countries with low rates of immigration to the US. Applying for the lottery is free and needs to be done on-line directly with the US State Department.

The current application period (DV2012) runs from noonEDT on Tuesday, October 5, 2010, to noonEDT on Wednesday, November 3, 2010. Note that this is a considerably shorter application window than in past years. The State Department’s detailed instructions for the electronic filing process are available at www.dvlottery.state.gov. Another change for this year’s lottery process is that winners will no longer be notified by mail. Instead, applicants must check the status of their entries on-line at that web site. In the past there have been scams involving false notifications of winning entries by post, email, or telephone. These scams seek a payment or financial information from the supposed winners. Remember that there will be no such communications from the US government. Their lottery web site will be the sole source of accurate information on winning entries.Some diversity visa lottery web sites may be outright scams, either promising to increase your odds of winning or just taking your money and providing no service in return. The only web site you need to consult is the official one at www.dvlottery.state.gov