A male, un-castrated pig is known as a boar -- a male pig that has been castrated after sexual maturity is known as a stag, or a hog -- and a pig that has been castrated before sexual maturity is known as a barrow hog, barrow pig or barrow.
The castration of a barrow pig is usually done on a very young shoat or piglet. This allows the procedure to be conducted while the pig is in a stage where it heals easily and is easier to handle. In most cases, the procedure is required to be carried out by a licensed veterinarian: However, for small scale castration, it is legal for anyone to perform the procedure, as long as they use humane methods.
After reaching sexual maturity, a boar becomes aggressive towards other males and must be kept in a separate pen. Once castrated, the barrow no longer feels these aggressive urges. Fully matured and un-castrated boars are also difficult for humans to handle and control. Once castrated, the barrow is usually more calm and tractable and is easier to transport and handle.
A fully matured boar that has not been castrated will develop an unpleasant odor and taste commonly referred to as "boar taint." This is caused by the testosterone and chemicals produced by the sexually mature boar in order to attract females. Removing the testes prior to maturation prevents the production of these chemicals.
In most cases, the castration of a pig will cause it to gain weight and grow more quickly. This phenomenon can be observed in most castrated animals. It is due to the lower level of hormones, which results in a slower metabolism as well as an increased desire to sleep. Because the barrow gains weight faster, it can be slaughtered and sold more quickly.