On Receiving Jesus in Holy Communion

"A person who is conscious of grave sin is not to celebrate Mass or receive the body of the Lord without previous sacramental confession unless there is a grave reason and there is no opportunity to confess; in this case the person is to remember the obligation to make an act of perfect contrition which includes the resolution of confessing as soon as possible." "A person who is to receive the Most Holy Eucharist is to abstain for at least one hour before holy communion from any food and drink, except for only water and medicine.”

Catholics may receive Communion during Mass or outside of Mass, but "a person who has already received the Most Holy Eucharist can receive it a second time on the same day only within the eucharistic celebration in which the person participates", except as Viaticum (Code of Canon Law, canon 917).

In the Western Church, "the administration of the Most Holy Eucharist to children requires that they have sufficient knowledge and careful preparation so that they understand the mystery of Christ according to their capacity and are able to receive the body of Christ with faith and devotion. The Most Holy Eucharist, however, can be administered to children in danger of death if they can distinguish the body of Christ from ordinary food and receive communion reverently" (Code of Canon Law, canon 913).

In the Eastern Catholic Churches, the Eucharist is administered to infants immediately after Baptism and Confirmation (Chrismation).   Holy Communion may be received under one kind (the Sacred Host alone), or under both kinds (both the Sacred Host and the Precious Blood). "Holy Communion has a fuller form as a sign when it is distributed under both kinds. For in this form the sign of the eucharistic banquet is more clearly evident and clear expression is given to the divine will by which the new and eternal Covenant is ratified in the Blood of the Lord, as also the relationship between the Eucharistic banquet and the eschatological banquet in the Father's Kingdom... (However,) Christ, whole and entire, and the true Sacrament, is received even under only one species, and consequently that as far as the effects are concerned, those who receive under only one species are not deprived of any of the grace that is necessary for salvation" (General Instruction of the Roman Missal, 281-282).

"The Diocesan Bishop is given the faculty to permit Communion under both kinds whenever it may seem appropriate to the priest to whom, as its own shepherd, a community has been entrusted, provided that the faithful have been well instructed and there is no danger of profanation of the Sacrament or of the rite's becoming difficult because of the large number of participants or some other reason" (General Instruction of the Roman Missal, 283).

In Eastern Catholic Churches the Eucharist is always received under both species (bread and wine), as was done at Mass also in the West until the opposite custom came into use, beginning in about the twelfth century.

With the change from receiving the Eucharist under both kinds to receiving under the form of bread alone, it also became customary in the West to receive the Host placed directly on the tongue, rather than on the hand, but this was prescribed neither by the Roman Missal nor by the Code of Canon Law. Since the late twentieth century many Episcopal Conferences allow communicants (at their personal discretion) to receive the Host on the hand, except when Communion is distributed by intinction (partly dipping the Host in the Chalice before distributing it).

The General Instruction of the Roman Missal, 118 mentions a "Communion-plate for the Communion of the faithful", distinct from the paten, to prevent the Host or fragments of it falling on the ground.

Reference:  http://en.wikipedia.org/wiki/Eucharist_in_the_Catholic_Church