He remains a priest forever, but his faculties to function as a priest may be suspended. This may be as a censure for serious misdeeds, or voluntary. Suspension means that he cannot say mass or adminitster the sacraments [except in case of emergency - he can hear the confession of a dying person for example.] Suspension may also remove many of his obligations as a priest. He will usually be forbidden to take any role, such as lector, EMHC, server etc which might confuse others as to his status. Depending on the circumstances he may be allowed to marry. I think this is usually granted only in cases where the suspension was voluntary and involved no wrong doing.
This is a collection of bits and pieces that I have picked up from various sources and instances. It is by no means complete or authoritative - use at your own risk.