Fantasy Football Definition of "Free Agents"
A "free agent" in NFL football is a player who isn't under contract to an NFL team. A restricted free agent is where this NFL player's former team retains some rights to resign this player, while an unrestricted free agent is where this free to sign with any team in the NFL. Fantasy football borrows the term "free agent" to describe any NFL player who is not on the roster of any fantasy football team in the league.
Because fantasy football free agents are not a part of any team, generally any team in the league can add that player to their roster, if they so choose. Every league has their own rules to regulate the free agency process. Here is a list of the major types of free agency in fantasy football.
Waiver Wire Free Agents
Most fantasy football leagues have a "waiver wire", also known as the "add/drop" process. When you want to add a free agent player to your roster, you make a waiver claim through your league's waiver wire process software. Generally, you select the player and click to add him to your team. Because most leagues have a rigid roster limit, you have to drop a correspondingly number of players for every free agent you add. That is why the process is called add/drop, because every free agent addition implies a drop.
How and when the waiver wire works is a major rules decision in fantasy football.
Weekly Waiver Wire Priority List
Many leagues have a waiver wire priority list. Teams make waiver wire claims on the players they want, but these do not happen immediately. Instead, there is a set time every week, usually in the middle of the fantasy football week (Tuesday or Wednesday, but definitely before 8pm Thursday, when games sometime start) where these free agents are assigned to teams. Every team in the league has a position on the priority list, usually with the team with the worst record receiving 1st priority, and the team with the best record receiving last priority.
Blind Bid Waivers
Leagues which don't like the idea of one team getting priority because they had a bad draft might choose to have a blind bid waiver wire process. In this form of free agency, teams use the league website to assign a (fake) money value to the free agents they want (with everyone having the same amount of dollars to spend). The team making the highest bid wins that free agent, but has less money to spend on subsequent free agents. Like the waiver wire priority list, blind bids are generally processed in the middle of the week, allowing teams time to make other roster moves and change their starting lineups accordingly. Blind bids can be processed automatically by the league site, but are usually processed by the league commissioner himself.
First-Come, First-Served Free Agency
Some leagues simply have first-come, first-serve free agency. If you want a player, go to the site at any time in the week and add that free agent. You'll have to drop someone from your roster to do so, but otherwise, there are no restrictions. First-come, first-serve free agents introduces the possibility that someone will pick up a backup while watching a game where a starter has a knee injury, or during the week when breaking news happens, or a newbie owner drops really good players after one bad week (with a rush by the veteran teams to add that player first). While there are chances for season-changing moves any time or day or night, in a mature, well-informed league, first-come, first-served free agency tends to work just fine.