A green card, also known as a greencard, is a document that identifies someone who is not a US citizen but has been given permission to live in the US permanently - it is also called a US Permanent Resident card. At one point, the green card was actually green, but, like the used-car guide known as the 'blue book', it has been various colors throughout the years. Today's green card is white.
The permanent resident non-citizen must carry his or her green card at all times since it may be necessary to prove that they are in the country legally. It can also be used to prove residency status when applying for work. Non-permanent resident aliens need an Employment Authorization Document to be legally hired by a US employer, but the holder of a green card has implicit employment authorization.
The green card is for many the first step on the road to naturalization and full US citizenship, and is highly sought after. The official green card has a number of anti-forgery features to prevent it from being easily duplicated. Getting a green card, or permanently immigrating to the US, can be a lengthy process, and not everyone can qualify for one. You can obtain a green card if you are closely related to a US citizen -- a spouse, parent, child or sibling. If you have a job offer within the US, it can help you qualify for immigration and permanent residency through employment.
One unique qualification for permanent resident status and a green card is through investment. If you are willing to invest in creating a business in the US that will employ at least ten people, you may qualify for a green card. You will be required to prove you have the necessary capital for the enterprise, of course. If you are from a country with a low rate of immigration to the US, you may take part in a lottery that authorizes 50,000 permanent residency visas a year.