In European soccer, players are not traded the same way that players are in America. Instead players are transferred when a club negotiates a deal with another club to pay for their rights. This usually then means that the club will also need to negotiate contract terms with the player, however this has usually been determined before the transfer goes through.
There are also specific times when players can be transferred. This is known as the transfer window. The transfer window is opened at two times during the season.
In some cases a player will decide that they no longer want to play for a club and will request that they be transferred. In some cases this works out, in other cases the club may not want to let them go and may keep them at the club until their contract runs out. This can also be the case when a club offers a transfer fee that the selling club does not agree is high enough.
Nowadays a player can leave a club at the end of his contract and become a “free agent.” In Europe this is known as “leaving on a free,” basically meaning that they are allowed to go to another club without that club having to pay a transfer fee. It has also been called leaving on a “Bosman.” This is named for the Belgian player Jean-Marc Bosman who challenged the previous transfer rules in court after he was not allowed to leave his club at the end of his contract.
There is also a system known as “loaning.” In this a club can “loan” a player to another club. This is usually used for younger players to help get them experience or for players who do not have a regular first team place, but want to be able to play regularly, usually for a lesser team.
The club loaning out the player will often continue to pay their wages for the duration of the loan. The loan system is typically seen as mutually beneficial. The club receiving the player will usually be getting a player that can help their club (since this is usually done with lesser clubs taking loaners from a larger club). The loaning club has a player who would not normally get much playing time is able to gain experience with another club that they can bring back to their “home” club. Sometimes this player is later sold on to the team taking them on loan which again benefits both clubs in the deal.
There are also some difficulties in this system since the team that purchases the player basically owns their ability to play soccer by purchasing their registration to play. While they will also negotiate a contract there is still the chance that a team can decide a player is no longer needed and force them to play in a reserve team or train separately from other players.
Transfers in European soccer, and the Premier League, work much differently than they do for other sports, particularly American sports.