Salary Cap: Though it doesn't seem that way, the NFL has a hard salary cap of $75.007 million. Teams must be under the cap from Feb. 28 through the remainder of the year. The salary cap is calculated by adding the base salary with any roster or reporting bonuses, a prorated part of the signing bonus and any incentives that are considered likely to be earned.

Pro-rated signing bonuses: This is the hardest thing about the cap to understand. Signing bonuses are spread for cap purposes over the length of the contract. For example, a $4 million signing bonus spread over a four-year contract counts $1 million against the 2003 cap. This explains why a team can pay a player $5 million in one season and it counts only $2 million against the cap. You add the $1 million base with the 2003 pro-rated part of the signing bonus and you have a $2 million cap number.

Salary cap hits: The 2003 pro-rated part of a signing bonus is always going to count in that year's cap. But, because every dollar counts in the salary cap, teams must add the remainder of a signing bonus if they cut or trade a player. They do this in two ways. If a player is cut before June 1, the cap hit is this year. Take that $4 million signing bonus example. If that player is cut before June 1, the $1 million portion counts this year but the remaining $3 million must also count this season. Thus, that $5 million contract must all count in 2003. But if the player is released after June 1, the team can delay taking the cap hit until next year, 2004. That's why $1 million will be on the books this year, but $3 million goes against the 2004 cap.

Why trades aren't happening: Often, it's because of that pro-rated signing bonus hit. Unlike releasing a player, there is no way that a team trading a player can delay taking a salary cap hit. Once a player is traded, the remaining pro-ration is moved into that season. The same standard applies if a player released on waivers is acquired in a waiver claim. Through most of the year, this only applied to players who haven't qualified for unrestricted free agency -- those players who haven't been through four NFL seasons. Waiver claims move up the cap hit immediately. The trading deadline begins Feb. 28.

Unrestricted free agency: These are the players who are completely free and can sign with any team.

Restricted free agency: From Feb. 28 through a week before the draft, players with three years of experience can shop their services, but teams that sign them may have to pay a price related to which round the player was picked in when they came into the league. For example, a former third-round choice requires third-round compensation from the signing team. Teams that are losing a player, though, have a week to match a restricted offer sheet. Restricted free agents are offered a one-year tender at $619,000. Teams can further restrict movement of these players by increasing the tenders. By offering $1.35 million, the team can't lose that player unless they receive a first-round choice. For $1.801 million, the team puts the player's price at first- and third-round choices. A more exclusive tender has been created to prevent movement of a player with three years experience. It's rarely used, but it would count $2.301 million against the cap.

Transition tags: Takeo Spikes of the Bengals received the transition tag by the Bengals. That means he was offered the average of the top 10 salary cap numbers at his position in a one-year tender. He can bring back any offer to the Bengals and give them seven days to match. If the Bengals don't match, Spikes moves without draft compensation.

Franchise tags: These are more restrictive. Teams offer players the average of the top five cap numbers at their position to restrict their movement. If a team signs a franchise player, they may have to give a team two first-round choices or work out a trade satisfactory to that team. One problem with the franchise tag is that teams like to keep them year in and year out. If a team re-signs their franchise player before July 15, they lose the ability to franchise future players for the length of the contract. What teams do is sign the player to the one-year offer and then renegotiate an extension after July 15. Teams who use the franchise tag have until the middle of March to reach an agreement to a long-term deal, and if they do, the team will get the ability to franchise a player again for the next season.

Minimum salaries: Here is the league wage scale -- rookies $225,000, second-year players $300,000, third-year players $375,000, fourth-year through sixth-year players $450,000, seventh-year through ninth-year player $655,000 and those 10 years or more experience, $755,000.

Cap Relief For Veterans: In 2002 and continuing this year, the league has created a cap relief category that allows teams to sign players with more than four years of experience and have those one-year contracts count for only $450,000 under the cap. Let's say a 10-year veteran sign a one-year deal. He will make $755,000, but the team that signs him only has to count $450,000 against its cap.

John Clayton is a senior writer for ESPN.com.