The NFL first instated a salary cap of $34.6 million for each franchise in 1994. Since then, the NFL's salary cap has increased considerably. The salary cap for the year 2014 was $133 million.
The NFL's salary cap serves two somewhat interrelated purposes. First, it prevents wealthier teams from dominating the league by spending more money to sign top players. Second, it keeps costs for each team down, since no team is allowed to spend more on player salaries than the cap number. The salary cap itself is based on league revenues, so it tends to increase with inflation and with the NFL's success.