In medieval Catholicism, relics were usually the physical remains of a saint, the founder of a religion, or some other highly venerated person of religious importance. These remains could be all or part of the person's skeleton (most commonly a finger, but often a hand, foot, tooth, or even skull); they might also be fingernails or locks of hair. A relic might also be something the saint had used or touched; usually, scraps of garments the venerated person was believed to have worn, but sometimes a tool (or, more often, a portion of a tool) the saint had used in his work.
One of the most famous relics of the Middle Ages was the True Cross, a piece of wood from what was believed to be the cross on which Jesus had been crucified. During the First Crusade, a piece of iron believed to be the spearhead of the Holy Lance -- the spear that had pierced Christ's side on the cross -- was discovered in a cathedral in Antioch. Of course, the Holy Grail was the much-sought-after, and never found, relic that Jesus had drunk from at the Last Supper.
Just how authentic any relic really could be is up for debate. If the deceased person had only recently passed on, it was not unlikely for someone to acquire a portion of that person's body before he or she was laid to rest. This may strike us today as unreasonable, but some medieval people requested portions of their bodies to be placed somewhere other than with their interred corpses. For example, when Richard the Lionheart died, he asked that his heart be buried at Rouen, his brain and entrails be sent to an abbey in Chattoux, and the rest of his body be laid to rest in Fontevraud. His wishes were carried out.
If a church was fortunate enough to acquire the relic of a saint or other religious person, both the church and the town in which it stood were likely to benefit, especially if the person was popular. The church that housed a relic would usually become the object of pilgrimage, and as making a pilgrimage was something most Christians tried to do at least once during their lifetimes, (and some liked to do several times), the town would see increased trade. Also, devout individuals who lived in the town would feel a certain degree of comfort and pride at having the venerable object in their midst.
Reliquaries were built to house relics. A reliquary would be constructed with specific features designed to accommodate the particular aspects of the relic it contained. They were very often fashioned from finely-worked wood, or sometimes from bone or ivory. Glass, quartz, semiprecious stones, even precious gems were used to ornament a reliquary, which might be embossed in copper, bronze, or even silver and gold. If the relic were small enough, the reliquary might be made of glass or quartz. Enamel was frequently used to decorate reliquaries.
A reliquary was usually portable, so that it might be borne in processions where everyone might see it. Sometimes, a reliquary would look like a small-scale church or chapel, or perhaps a sarcophagus. Other reliquaries were designed to look like the object it contained: a finger, hand, foot, or other body part. Occasionally, it was made in the form of a statue or bust of the person whose relic it held within.
Quite a few relics and reliquaries remain from the Middle Ages, most of which still hold religious significance for devout Catholics.