US Permanent Residents are often referred to as “green card” holders. US Permanent Residents enjoy many of the same benefits as US citizens except that they are subject to certain grounds of deportation and are unable to vote in certain elections.
The other major difference between US citizens and Permanent Residents is how they are treated when they leave the US for long periods. In general, it is very difficult for a US citizen to lose their American citizenship by the mere fact that he or she is no longer living in the US. See the link on loss of US citizenship for more information.