A reliquary is where holy relic
s are kept. Usually its some box or a casket even, and the relic isn't always visible. Some relics include pieces of the original cross
, portion of a saint's remains, or an article that is associated with a saint
Typically, inside or under the altar of a Catholic church there is some form of relic, so the altar is considered a reliquary.
The Catholic Church has developed a method of classification of relics based on their characters. This classification system has its limitations, but overall is an excellent aid in describing these relics. It is divided into three classes, as follows:
First Class Relics: Remains of the bodies of saints, for example bones, hair, or blood, as well as the instruments of Our Lord's Passion.
Second Class Relics: Articles associated with the lives of the saints, such as clothing, books, and anything else the saints used in life.
Third Class Relics: Items, usually small pieces of cloth, which have come into contact with First or Second Class Relics and which are then treated themselves as relics. Among the first third class relics were small pieces of cloth lowered through holes in the altars of churches built over martyr's tombs or placed on the slabs covering them.
As of right now, Mother Teresa's blood is on display at a reliquary in Rome. The blood is part of the "first class" category.
An example of some reliquaries are availible at http://www.geocities.com/Athens/Olympus/9587/relics.html