[Schneed10]

Exclusive-rights free agents
Exclusive-rights free agents (ERFAs) are players with two or fewer seasons of service time and whose contracts have expired. If their team tenders a qualifying offer (a one-year contract usually at league-minimum salary) the player has no negotiating rights with other teams, and must either sign the tender with the team or sit out the season.[5]

Restricted free agents
Restricted free agents (RFAs) are players who have three or fewer accrued seasons of service and whose contracts have expired. RFAs have received qualifying offers from their old clubs and are free to negotiate with any club until a deadline which occurs approximately a week prior to the NFL Draft (for 2010 the deadline was April 15), at which time their rights revert to their original club. If a player accepts an offer from a new club, the old club will have the right to match the offer and retain the player. If the old club elects not to match the offer, it may receive draft-choice compensation depending on the level of the qualifying offer made to the player.[6]

Unrestricted free agents
Unrestricted free agents are players with expired contracts that have completed four or more accrued seasons of service. They are free to sign with any franchise.[6]

Undrafted free agents
Undrafted free agents are players eligible for the NFL Draft but who are not selected; they can negotiate and sign with any team.

So Chris Thompson actually has 4 seasons under his belt, but he only officially accrued three NFL seasons because he was hurt so much for one of them (you have to be on the roster or practice squad for 6+ games in a season to have an accrued season, and not on IR during that time). So Thompson only has 3 accrued seasons right now, making him an RFA instead of a UFA.