NFL rookie contracts are generally four years long. However, players who were selected in the first round of the NFL Draft have a team option to be extended for a fifth year.'
The flexibility of keeping a high-end player on a relatively cheap deal for an extra year is one of the key details that make first-round draft picks so valuable in the NFL. (The other, of course, is the quality of the players who are still available near the beginning of the draft.)
This value is especially relevant to NFL teams at the quarterback position, where an extra year for only a couple million dollars could be the difference between winning the Super Bowl and missing the playoffs.
Free agents who are signed but not drafted generally agree to slightly shorter three-year deals, per NFL rules laid out in the current collective bargaining agreement.