Sainthoods were bestowed by the Catholic Church upon to those who they wished to recognize posthumously for the power of their faith and the miracles associated with them. The Church would canonize a Saint if two miracles could be attributed to that person after death. If only one miracle was proven that person would be beatified, as in the case of Blessed Herman the Cripple.
For this reason, churches and the nobility like to possess various relics and artifacts that were claimed to have belong to the saints, including bodyparts. Of course, there were a lot of false or incorrect claims made. At one stage Martin Luther asked how the 12 apostles could be buried in 26 places in Europe. The shroud of Turin may in fact be a medieval fake.
The Catholics also liked to make use of established local or pagan religions when trying to convert an area. One way was to take a popular local spirit or diety and transform it into a Christian saint. So Ireland's Brighid became Saint Brigid, and any annual ritual date became the saint's feast day. Even the Buddha became known as Saint Josaphat, although he was later desanctified by the Catholic Church.
Various skills, crafts and trades have patron saints that are considered to give them protection and guidance. These saints have some link to the field for which they are a patron. This link is sometimes very weak. For instance, the apostle Nathaniel (aka Bartholomew) is a patron saint of tanners and he is often shown holding a skin. The fact that he was flayed alive and it is his skin is not always obvious but it does explain why his body is sometimes painted red.
Literature that discusses the life of one or more saints is a hagiography.
In the SCA, the names of saints are included in the names of colleges as their patron saints. This is to reflect the fact that many medieval and Renaissance colleges were also named after patron saints. There are also numerous examples of saints being spontaneously created for entertainment value, such as Saint Brand the Maille-Maker or Saint Cunard the Tenacious, in the SCA. While widespread, these practices do not break the rule on religion in the SCA.