American green card is an official license granted by the US government for people seeking a lawful permanent residence in the US. It is just a step short of obtaining a US citizenship. It gives the right to live and work permanently in the United States.
It was earlier called the Resident Alien Card and was green in color. It is officially called the form no I-551.

The American green card can be sought on the basis of employment, marriage, investment, relatives, adoption, or through green card lottery program. There are also other options such as refugee based or asylee based.
There are several key benefits of American green card, which includes the right to enter or leave the country at will without any complications at immigration check. You can shift between any parts of the US territory permanently, can work in any company within the American territory or even start your own business or run a corporate system. It provides security clearances, which means that you have greater job opportunities.

It also gives the holder the social security advantages when you retire provided you have worked for at least 10 years. Availing insurances, owning property, buying cars, etc would become easy.
You can avail government sponsored financial aid for education and pay less tuition fees to the university or institution. You can be exempted from export restrictions.
You can even carry out political campaigns. Above all you are free to exercise all the rights that an American does except for the right to vote and to hold some positions in the Federal bench.
The process of acquiring an American green card is a lengthy and time consuming procedure. It starts from applying for an AOS (Adjustment of Status) or consular processing depending upon the route that you would like to adopt.
Then it demands several documents such as fee payment, work permit or EAD, birth certificate, police clearances, travel document, biological information sheet, photographs, evidence of status, etc. medical examinations, finger prints and interview are other steps.

In case you are traveling abroad for long period you may need a re-entry permit.
In case of theft, destruction, mutilation or loss of the American Green Card a police report must be filed to avoid complications.

There are possibilities that you may lose your green card if you behave illegally. Retaining of green card is essential to enjoy the privileges throughout your life.