Transfers
Except in MLS and (sort of) Liga MX, there are no trades in soccer like there are in typical American sports. When a player signs a contract with a club, it is not transferrable. Therefore, players move clubs in the following ways:
1. Free transfer, which in Europe is also known as a
Bosman transfer, named after an insignificant Belgian player named Jean-Marc Bosman who couldn't move to a small club in France because they wouldn't pay a transfer fee for him even though he was out of contract. The Bosman case was basically the end of the European soccer version of the reserve clause. Overall, this is the way the majority of all players move, although that majority is weighted towards the lower levels of the sport. In the EPL and other large leagues, most players brought in come in on transfer fees. But when you are talking about a lower-tier Swiss league team, for example, they're going to save their expenses for wages and not spend money on transfer fees when there are plenty of players suitable for their level on a free. Also, lots of smaller clubs particularly in small countries simply don't have much money.
2. Transfer fees. Club A is interested in Player X, who plays for Club B. Club A can approach Club B with a transfer fee, which is essentially compensation for letting Player X out of his contract early. If Club B accepts the offer, Club A can negotiate a contract with Player X. That's the process. Most leagues have "transfer windows" when teams are allowed to acquire players. However, all across the world there's always a transfer window open somewhere. For example, MLS teams can buy players in March, but not English teams. So an MLS team could buy a player from an English club at that time, but not vice versa.
Some other random things:
* Often, players will have a "release clause" in their contract. This means that if another club offers a certain amount of money, the player's club is required to accept the offer. For example, Bayern bought Mario Götze for €37m because that was the release clause in his contract with Borussia Dortmund. At that point, there was little Dortmund could do. They could in theory offer him a larger contract and hope he would take it and stay, but Bayern has more financial firepower and more prestige, so Götze was always going to go.
* Larger clubs can try to "unsettle" players with transfer talk. A transfer with a fee almost always results in a larger contract. Why would you sign with a new club if it's a pay cut? So when a bigger club starts honing in on a player who is ready to make a jump in salary and club prestige, the smaller club feels forced to sell so that they don't have an unhappy, unmotivated player. The smaller club also doesn't want to develop a reputation for being a place where players can't progress, as that could pose a problem when trying to attract young and ambitious players.
3. Loans. Sometimes a player will join a new club on loan. His old club still holds his contract, but allows a new club to use him for a period of time. Often a loan period can be a full season or a half season or sometimes just a month. Loans are usually done for several reasons.
* The first is developmental. When a young player has outgrown reserve leagues (which are often a little weak) but isn't ready for first-team soccer at the club, he might be loaned out to a lower level. This is especially common in Italy, where this practice has essentially taken the place of any reserve league or structure. It's also fairly common in England. In Germany, Spain, and France, clubs maintain reserve teams at the lower levels of the soccer pyramid and young players go on loan less frequently.
* The second is salvaging value from a crappy contract. Let's say a club made a bad signing and no longer has use for a player. They can't sell him because he's not worth the contract he has and nobody would be willing to pay as much or more. One way to mitigate this sunk cost is to loan him out. While developmental loans are frequently free for the lower-level club, these loans can involve a small fee or, more likely, the new club agrees to pay a percentage of the player's salary. The player gets paid his full amount, the original club gets out from some of the player's cost, and the new club gets the player at a more reasonable amount.
* Loans also come with various stipulations. Beyond the aforementioned loan fee (which often is $0) and the way the player's wages are cut up, often there are rules. These include a recall clause (player can be summoned to the mother club at any time), a clause stating the player can't play against the mother club (if they are in the same league) or that a player can't play in the cup to prevent him from being cup-tied (if the clubs are in the same country), and so on.