Unrestricted free agents are players without a team. They have either been released from their club, had the term of their contract expire without a renewal, or were not chosen in league's draft of amateur players. These players, generally speaking, are free to entertain offers from all other teams and to decide with whom to sign a new contract.
The specific rules of restricted free agency vary among the major professional sports, but in principle, it means that a player is free to solicit offers from other teams for new contracts. However, before this player is allowed to sign with the new club, the current club has a chance to match (or come within 10% in some leagues) the terms of the new contract in which case the player must remain with the original team.
Age and experience in the league are the two most common criteria for whether a player qualifies for unrestricted free agency.
In the North American professional leagues, any player without a contract becomes a free agent. An Unrestricted Free Agent is able to sign with any team. A restricted free agent is only allowed to get offers from other teams if the player is offered by his own team. Usually, the better offer wins.