Green card is an identity card issued by US government, issued for non-citizen i.e., permanent residents. It have same benefits as US citizen working, opening business, leave and re-enter to US without any re-entry permit but only difference
Green card holder don't have right to vote.
You can get Green card based on your employment, with labor certification(proving shortage of US citizen for particular vacancy). Green card based on marriage, while US citizen petition for it or Family based Green card. Or winning a lottery program organized by US, almost 55,000 Green card for Alien. Learn How to get a Green card(DVD), with this you can easily understand the process.
If Green card is lost or stolen, it is very important to
renew green card quickly as possible. The Form I-90, is used to renew or replacement of existing green card.