A capon is a rooster that has been castrated before reaching sexual maturity.
The reason a rooster is made into a capon has mostly to do with the quality of the meat. But also, a capon is less aggressive than a rooster and is easier to handle.
Capon meat is tender and flavorful, compared with rooster meat, which can be quite gamy. Capon meat is also relatively fatty, and has a high proportion of white meat. The difference between capon meat and rooster meat is due to the absence of sexual hormones.
The process of making a rooster into a capon is called caponization. A capon is usually castrated at around 8 weeks of age or earlier. Capons are generally slaughtered at around 10 months of age or younger (as compared with around 12 weeks for a regular roasting chicken).
Usually capons are roasted, and the procedure for doing so is really no different from roasting a chicken.
Traditionally, roosters are braised (the classic French dish coq au vin involves braising a rooster in red wine), because their meat is tougher than chicken meat, and they are usually slaughtered at an older age, which makes the meat tougher.
As such, braising is also a good cooking technique for preparing capon. Also see: Braised Chicken Recipe