On Tuesday, October 1, 2013 at 12:00 noon, Eastern Daylight Time (EDT), the U.S. State Department's website will open for people from certain countries (the ones that have sent the smallest number of immigrants to the U.S. in recent years) to apply for the Diversity Visa Lottery (DV-2015). The application period will close on Saturday, November 2, 2013 at 12:00 noon, Eastern Daylight Time (EDT).
This year's visa lottery is little changed from that of previous years. As before, the person must make sure their country is on the list of eligible ones -- Nigeria dropped off this year's list. Applicants must show a certain level of education, and can only enter once, via an online application. Those who win can apply to bring a spouse (now including same-sex spouses, if legally married according the the state or country where the wedding was held) and children, as well.
For details on the basics, see one of Nolo's books covering this topic (U.S. Immigration Made Easy or How to Get a Green Card) or the "Diversity Visa Lottery Green Cards" section of Nolo's website. And to access the State Department's detailed instructions and application portal, see the "Diversity Visa (DV) Program" page of its website.