This is a couple of years old but it gives you the basics of the salary cap. From nfl.com .
The 2015 NFL salary cap is confirmed at $143.28 million. Under the Collective Bargaining Agreement, each club has the option to carry over unused cap space from the prior League Year. Below is every team's adjusted salary cap position given its carryover amount and other adjustments from the 2014 season.
2015 NFL Salary Cap and Adjusted Team Positions - NFLPA
https://www.nflpa.com/.../2015-nfl-salary-cap-and-adjusted-team-positions