There are several types of free agents in the NBA, and this list covers them all.
UFA: An unrestricted free agent who is able to sign with any team.
RFA: A restricted free agent who is allowed to field offers from other teams, but his current team has the right to match.
Player: He can become a free agent by declining the player option in his contract.
Club: He can become a free agent if his team declines to excercise its club option.
Bird: If a player has "Bird rights" it means his current team is allowed to sign him for more than the maximum NBA contract without the overage counting against its cap.