The offseason truly starts today, as teams are finally allowed to actually do something. In this case, they can apply the franchise tag to free agent players. If you want to know how the rest of the offseason plays out, here's a quick calendar (thanks to The Sideline View for many of these dates) :
February 20: Opening of period to designate franchise/transition players.
February 22-28: NFL Scouting Combine in Indianapolis.
March 5, 4:00 PM: End of period to designate franchise/transition players.
March 13, 4:00 PM: Start of free agency, trades.
March 13, 4:00 PM: Latest possible date for start of restricted free agency signing period (actual starting date not clear). Teams must have tendered their restricted free agents by this date to retain a right of first refusal and a right to draft choice compensation. Other teams can offer contracts to other teams' restricted free agents as of this date.
March 13, 4:00 PM: Exclusive rights free agents must be offered a minimum salary contract by this time for teams to retain exclusive negotiating rights.
March 26-28: Annual league meeting.
April 1: Beginning of offseason workout program for teams with new head coaches.
April 16: Beginning of offseason workout program for other teams.
April 21: Likely end of restricted free agency period, teams can no longer offer other teams' restricted free agents contracts. This date might fall earlier depending on an agreement between the NFLPA and NFL. The CBA states this is the latest date for restricted free agency to end.
April 25: Last date for teams to match offers made to their restricted free agents by other teams.
April 26: NFL Draft, first round.
April 27: NFL Draft, second and third round.
April 28: NFL Draft, fourth through seventh round.