The things which help us most are the things which appeal to our human nature. An angel, being wholly spiritual, can be perfectly satisfied with those things which are entirely spiritual. Man is made of body as well as soul; and he gets his knowledge through the senses of the body. Therefore, if he occupies himself with things which are wholly spiritual, his human nature is under a severe strain. God, Who made him, is well aware of this. That is one reason why God became man; for it is much easier for us, with our human hearts of flesh and blood, to love God in Jesus Christ, than to love Him when we contemplate Him as a pure Spirit. In the same way, when Jesus Christ was planning to give us means to aid us in attaining our salvation, He chose the means that would help us most as human beings; as creatures of flesh and blood as well as spirit.

These means, and which Jesus Christ instituted as aids to our spiritual life and channels of grace, are called Sacraments. Like ourselves, the Sacraments are neither wholly spiritual nor wholly material, but a mixture of both. What is material in the Sacrament appeals to us through our bodily senses, and helps us to realize in some way, what the spiritual part of the Sacrament does for our souls. Sacraments are instituted by Christ and entrusted to the Church, as they are the actions of Christ and His Church. They stand out as the signs and means by which the faith is expressed and strengthened and therefore contribute in the highest degree to the establishment.

What a Sacrament is and what it does.
The Sacraments instituted by Christ are the Sacraments of the New Testament. The rituals of the old Law did not confer grace, rather the faith in God. The sacraments of the old Law signified grace to come. They were signs of faith. The Sacraments of the New Law not only signify grace but also confer grace.
A Sacrament is an outward sign, instituted by Christ to give grace. They are sensible signs by which invisible grace and inward sanctification are communicated to the soul. A Sacrament is a visible form of an invisible grace.

What constitutes a Sacraments?
Each Sacrament has four elements: 1. Form, 2. Matter, 3. Subject, 4. a Minister.
Form (Formula) is the words that are spoken. Matter (Material) being used and accompanying gestures. Matter and form make up the signs. Form must be spoken by the same person doing the action. Form and Matter correctly performed makes the Sacrament valid. The subject is the person receiving the Sacrament. The Minister is the one who effects or ministers it. The Sacrament is not brought into being if any one of these three requirements are lacking. (i.e., if there is lacking the due form of the words, if the matter is not present, or if the minister does not intend to confer the Sacrament.)
  
Each Sacrament gives to the soul not only sanctifying grace, but also actual graces - lights and helps to do good. The number of Sacraments instituted by Christ is seven. They are Baptism, Confirmation, The Holy Eucharist, Penance, Anointing of the Sick, Holy Orders and Matrimony.

Source: Rev. W. Frean, Commentary on the Catechism.

Certain Sacraments imprint on the soul a character which is a certain spiritual sign distinct from the other Sacraments. Such are the Sacraments of Holy Orders, Baptism, and Confirmation. A Sacrament which imprints character on the soul is never repeated in a person who has once received it. Thus, he who is baptized need never again receive this Sacrament. Neither can he who has been confirmed receive Confirmation again, nor he who has been ordained repeat his ordination. The reason is that the character which each of these Sacraments impresses is indelible (incapable of being removed.) The interior disposition of the recipient plays a role in each Sacrament. Thus, the effect of the Sacrament can be impeded (obstruct or blocked) through the fault of the recipient, such as when a person feigns to receive it but has a heart unprepared to receive it worthily. Although he actually receives the Sacrament, such a person does not receive the effect of the Sacrament: the grace of the Holy Spirit. "For the Holy Spirit of discipline will flee from the deceitful." (Wisd. of Sol. 1:5). On the other hand, there are some who never receive sacramentally, yet who receive the effect of a sacrament because of a devotion toward the sacrament which they may have in desire or in a vow.

Source: Saint Thomas Aquinas, God's Greatest Gifts.