A United States Permanent Resident Card (USCIS Form I-551), formerly Alien Registration Card or Alien Registration Receipt Card (INS Form I-151), is an identification card attesting to the permanent resident status of an alien in the United States. It is known informally as a green card because it had been green in color from 1946 until 1964, and it has reverted to that color since May 2010.
Green card also refers to an immigration process of becoming a permanent resident. The green card serves as proof that its holder, a lawful permanent resident (LPR), has been officially granted immigration benefits, which include permission to reside and take employment in the United States. The holder must maintain permanent resident status, and can be removed from the United States if certain conditions of this status are not met.
Green cards were formerly issued by the Immigration and Naturalization Service (INS). The Homeland Security Act of 2002(Pub. L. No. 107–296, 116 Stat. 2135) dismantled INS and separated the former agency into three components within the Department of Homeland Security (DHS). The first, the United States Citizenship and Immigration Services (USCIS) handles applications for immigration benefits. Two other agencies were created to oversee the INS' former functions of immigration enforcement and border security, Immigration and Customs Enforcement (ICE) and Customs and Border Protection (CBP), respectively.
Permanent residents of the United States eighteen years of age or older must carry their valid physical green card itself at all times. Failing to do so would be a violation of the Immigration and Nationality Act, carrying the possibility of a fine up to $100 and/or imprisonment for up to 30 days for each offense. Only the federal government can impose these penalties. (Source: Wikipedia)