The affiliations between the majors and minors are contracts that are drawn up to last anywhere from one to five years. The salaries of the minor league players are paid by the major league teams. All other costs and expenses are taken care of by the minors.

Affiliates signed by the major league clubs do change, due to financial and competitive changes, or perhaps due to a move. For instance, the Houston Astros, in an effort to improve its fan base to a wider area of Texas, switched their affiliation from the Zephyrs to Round Rock. The Zephyrs were forced to find another team to sign with in order to remain in the system of affiliation. Currently, the longest lasting affiliation has been between the Orioles and their Rookie-level Appalachian League, the Bluefield Orioles, which has been around since 1958.

An affectionate term for the minor leagues is the farm system, because the minors are used to groom and grow ball players to be picked for the majors when they are ripe. Some teams boast of a great farm system which is somewhat indicative of how good their team will be in the upcoming years. Of course, the worse a team is the better chance they have to draft players and therefore build up a better farm system. It can also be built by good trades.