The Green card system is for immigrants who want to become citizen.
"Who is a Green Card Holder (Permanent Resident)?
A Green Card holder (permanent resident) is someone who has been granted authorization to live and work in the United States on a permanent basis. As proof of that status, a person is granted a permanent resident card, commonly called a "Green Card." You can become a permanent resident several different ways. Most individuals are sponsored by a family member or employer in the United States. Other individuals may become permanent residents through refugee or asylee status or other humanitarian programs. In some cases, you may be eligible to file for yourself.