There are two occurrences for which a green card is taken from an individual. First is being convicted of a substantial crime. Committing a serious crime makes the person removable or inadmissible.
Secondly, the U.S. must remain the person's permanent residence. "Abandonment of residency" is an essential restriction on people with permanent resident status must follow. The main rule of this standard is the person is not allowed to travel outside of the U.S. borders for longer than 6 months without prior notification to the INS. Though permanent residents are free to travel outside of the U.S., the INS considers traveling for longer than 6 months as non-temporary. In order to avoid the consequences of abandoning your residency, set up a "re-entry permit" before leaving U.S. boundaries.
The notion green card holders have to return the United States once a year to keep their card active is a misconception. The INS looks at behavior of the foreigner with permanent residency as a means to determine whether he or she is abandoning their residency status. If the resident is gone for longer than a year, the INS will assume the resident is making another country their permanent home.
People who travel out of U.S. boundaries do not automatically have their green cards confiscated. If your intentions were to only remain out of the U.S. for a temporary amount of time may still be able to keep their permanent residency. Any future plans of traveling will require they use a special immigrant visa, or a reentry permit, can be acquired at a U.S. Consulate. They will not be able to use their green card as ID documentation for traveling into or out of the U.S.