To make certain that immigration into the United States is balanced, immigration officials in the government attempt to make green cards accessible to people who come from various different countries of origin. The manner in which this is done is known as the Diversity Immigrant Visa Program.
The Diversity Immigrant Visa Program provides up to 55,000 Diversity Visas every year to individuals from those countries that have always had a small historical presence in the United States. They are currently accepting applications for the 2015 lottery, so it is a good idea to try and find out if you may be eligible.
One of the main requirements is that you must have been born within a country that is accepted in the program. The place where you were born is more accepted than where you currently hold citizenship. Countries whose populace are not eligible for Diversity Visas include Brazil, mainland China, Dominican Republic, El Salvador, India, Mexico, Pakistan, Philippines, Great Britain/United Kingdom and dependant territories (excluding Northern Ireland), Vietnam, Bangladesh, Canada, Columbia, Ecuador, Haiti, Jamaica, Nigeria, Peru and South Korea.
That said, someone who was born in one of those countries can still put in an application for a Diversity Visa provided that their spouse comes from a country that is approved. You also need to have a high school diploma (or equivalent) plus two years of work experience that needed training, such as vocational school or college.