What Are Relics?
After the prophet Elisha died, he was buried in a cave. Sometime later, the body of another dead man had to be cast into the same cave hastily so those burying him could avoid a band of marauders. Then, ‘when the man came in contact with the bones of Elisha, he came back to life and rose to his feet’ (2 Kgs 13:20-21).
This story from 2 Kings provides a biblical example of a “relic,” which is an object connected with our Lord or a saint. Throughout biblical and Church history, relics have been venerated and have often demonstrated a capacity to convey the power of God through miracles, especially MIRACLES OF HEALING.
The Church divides relics into three classes. A first-class relic is a part of a saint’s body, as in the case of Elisha’s bones. A second-class relic is something a saint used during his life on earth, such as clothing. The Bible also records an instance of such a relic and its power: Elijah’s mantle, which parted the Jordan River after the prophet had gone to heaven: “Wielding the mantle which had fallen from Elijah, [Elisha] struck the water.... When Elisha struck the water it divided and he crossed over’ (2 Kgs 2:14). 
Third-class relics are objects that have been touched to a first-class relic. The Bible notes an example of this kind of relic, too, and the miracles it may work: “So extraordinary were the mighty deeds God accomplished at the hands of Paul that when face cloths or aprons that touched his skin were applied to the sick, their DISEASES LEFT THEM AND THE EVIL SPIRITS CAME OUT OF THEM (Acts 19:11.1 2)
We must keep in mind that the miraculous power conveyed through relics is not some kind of magic. It is simply God’s power acting through material means, analogous to the way he acts through the matter of sacraments and sacramentals - or, for that matter, the way he sometimes works miracles through the touch of a saint’s hands long before the saint’s death (see “Why Are Sacraments Necessary?” P.1468). 
Why would a saint’s relics be venerated? We might just as well ask why a woman would carry a lock of her beloved’s hair in a locket around her neck. The affection and honor shown a relic overflow from the affection and honor shown to the saints themselves, who are dear to us as exemplars of God’s grace, love, and holiness.
RELATED SCRIPTURE - Texts cited: 2 Kgs 2:14; 13:20-21 • Acts 19:11-12.
General: Ps 91:15; 112:1-9 • Mt 10:8 • Mk 16:17-18 • Acts 2:43; 3:1-13;
5:12-16; 8:1-8; 9:32-42; 14:3, 8-15; 16:18; 20:9-11; 28:8-10 • 1 Cor 12:28.

NIHIL OBSTAT: DOUGLAS K. CLARK, S.T.L. Censor Librorum                                               
IMPRIMATUR: +J. KEVIN BOLAND, D.D. Bishop of Savannah, January 15, 2005


The Relics of the Saints

236. The Second Vatican Council recalls that "the Saints have been traditionally honoured in the Church, and their authentic relics and images held in veneration"(323). The term "relics of the Saints" principally signifies the bodies - or notable parts of the bodies - of the Saints who, as distinguished members of Christ's mystical Body and as Temples of the Holy Spirit (cf. 1 Cor 3, 16; 6, 19; 2 Cor 6, 16)(324) in virtue of their heroic sanctity, now dwell in Heaven, but who once lived on earth. Objects which belonged to the Saints, such as personal objects, clothes and manuscripts are also considered relics, as are objects which have touched their bodies or tombs such as oils, cloths, and images.

237. The Missale Romanum reaffirms the validity "of placing the relics of the Saints under an altar that is to be dedicated, even when not those of the martyrs"(325). This usage signifies that the sacrifice of the members has its origin in the Sacrifice of the altar(326), as well as symbolising the communion with the Sacrifice of Christ of the entire Church, which is called to witness, event to the point of death, fidelity to her Lord and Spouse.
Many popular usages have been associated with this eminently liturgical cultic expression. The faithful deeply revere the relics of the Saints. An adequate pastoral instruction of the faithful about the use of relics will not overlook:
  • ensuring the authenticity of the relics exposed for the veneration of the faithful; where doubtful relics have been exposed for the veneration of the faithful, they should be discreetly withdrawn with due pastoral prudence(327);
  • preventing undue dispersal of relics into small pieces, since such practice is not consonant with due respect for the human body; the liturgical norms stipulate that relics must be "of a sufficient size as make clear that they are parts of the human body"(328);
  • admonishing the faithful to resist the temptation to form collections of relics; in the past this practise has had some deplorable consequences;
  • preventing any possibility of fraud, trafficking(329), or superstition.
The various forms of popular veneration of the relics of the Saints, such as kissing, decorations with lights and flowers, bearing them in processions, in no way exclude the possibility of taking the relics of the Saints to the sick and dying, to comfort them or use the intercession of the Saint to ask for healing. Such should be conducted with great dignity and be motivated by faith. The relics of the Saints should not be exposed on the mensa of the altar, since this is reserved for the Body and Blood of the King of Martyrs(330).

The word relic comes from the reliquiae, meaning “remains.”

A reliquary is a shrine that houses one or more relics.

Holy Relics of Sts. Franciso and Jacinta with Vice-Postulator's
Certificate installed at St Joseph's Cathedral, Raiganj Diocese West Bengal

Relic is in the center: piece of wood of the 2 Saints Francisco and Jacinta's coffin. This picture is from UK one, but similar to what is in Karjat Fatima Shrine and Raiganj Ctahedral