The current green card (United States Permanent Resident Card) is not actually “green”. It got its name from the early version that was green.
There is a difference between getting a green card and obtaining U.S. citizenship. A green card is normally issued for 10 years at the time and it must renewed upon expiration.
Individuals that get married to a U.S. citizen or a green card holder, or invest a substantial amount in the U.S. economy may apply for a green card with conditions that need to be removed after 2 years.
There are several ways to obtain a green card. The few lucky selected in the Diversity Immigrant Visa Program (a.k.a. Green Card Lottery) just have to go through the application process. However, only 55,000 individuals are given this opportunity every year.
Other ways of obtaining a green card can be through family in the United States or through an employer. Some categories have a limit of how many that can obtain a card annually (e.g. family based immigration), while other categories have no limit that can be issued (e.g. fiancés of U.S. citizens).
A green card gives an individual the right to legally live and work in the United States. Note that individuals with cards still hold the citizenship of their country of birth, but green card holders can apply for U.S. citizenship after certain amount of years as a green card holder. The United States allow individuals to hold dual or multiple citizenship.