The word relics comes from the Latin reliquiae (the counterpart of the Greek leipsana) which already before the propagation of Christianity was used to describe an object, notably part of the body or clothes, remaining as a memorial of a departed saint. In the minds of the early Christians, the bodies of the saints were transformed into “temples of the Holy Spirit.” (1 Corinthians 6:19) In the Middle Ages, relic veneration became obligatory and every church in Christendom was required to have a holy relic. Relics can include anything from the body parts of a saint to shards of the True Cross to pieces of cloth that have rubbed against a saint's tomb.

Generally, relics are divided into three broad groups called Classes:

First-Class Relics: Items directly associated with the events of Christ's life (manger, cross, etc.), or the physical remains of a saint (a bone, a hair, skull, a limb, etc.). Traditionally, a martyr's relics are often more prized than the relics of other saints. Also, some saints' relics are known for their extraordinary incorruptibility and so would have high regard. Parts of the saint that were significant to that saint's life are more prized relics. For instance, King St. Stephen of Hungary's right forearm is especially important because of his status as a ruler. A famous theologian's head may be his most important relic. (The head of St. Thomas Aquinas was removed by the monks at the Cistercian abbey at Fossanova where he died). If a saint did a lot of travelling then the bones of his feet may be prized. Current Catholic teaching prohibits relics to be divided up into small, unrecognizable parts if they are to be used in liturgy (i.e., as in an altar; see the rubrics listed in Rite Of Dedication of a Church and an Altar).

Second-Class Relics: An item that the saint wore (a shirt, a glove, etc.) Also included is an item that the saint owned or frequently used, for example, a crucifix, rosary, book etc. Again, an item more important in the saint's life is thus a more important relic.

Third-Class Relics: Any object that is touched to a first- or second-class relic. Most third-class relics are small pieces of cloth.

A Reliquary (also referred to as a shrine or by the French term châsse) is a container for relics. Reliquaries provide a means of protecting and displaying relics and range in size from simple pendants or rings to coffin-like containers, to very elaborate ossuaries. Many were designed with portability in mind, often being exhibited in public or carried in procession on the saint's feast day or on other holy days. Many reliquaries, particularly in northern Europe, were destroyed by Calvinists or Calvinist sympathizers during the Reformation, being melted down or pulled apart to recover precious metals and gems. Nonetheless, the use and manufacture of reliquaries continues to this day, especially in Roman Catholic and Orthodox Christian countries. Post-Reformation reliquaries have tended to take the form of glass-sided caskets to display relics such as the bodies of saints.

The earliest reliquaries were essentially boxes, either simply box-shaped or based on an architectural design taking the form of a model of a church; these were known as shrines or chasses. Chasse or "box reliquary" is usually of gilt-copper, decorated in champlevé enamel, with Limoges the largest centre of production .

Relics of the True Cross became very popular from the 9th century onwards and were housed in magnificent gold and silver cross-shaped reliquaries, decorated with enamels and precious stones. From about the end of the 10th century, reliquaries in the shape of the relics they housed also became popular; hence, for instance, Pope Alexander I's skull was housed in a head-shaped reliquary. Similarly, the bones of saints were often housed in reliquaries that recalled the shape of the original body part, such as an arm or a foot.

A philatory is a transparent reliquary designed to contain and exhibit the bones and relics of saints. This style of reliquary has a viewing portal by which to view the relic contained inside.

During the later Middle Ages, the monstrance was introduced—a form of reliquary which housed the relic in a rock crystal or glass capsule mounted on a rod, enabling the relic to be displayed to the faithful. Reliquaries in the form of jewellery also appeared around this time, housing tiny relics such as pieces of the Holy Thorn.

Altar reliquaries, encountered in several different forms, were traditionally embedded in altar stones under sealing wax.

Canon law requires the authentication of relics if they were to be publicly venerated. They have to be sealed in a receptacle and accompanied by a certificate of authentication, signed and sealed by someone in the Congregation for Saints or, today, by the local bishop where the saint lived. Without such authentication, relics are not to be used for public veneration.