Here are a few things the Catholic Encyclopedia has to say about the Virtue of Chastity:

As a virtue

Chastity is the virtue which excludes or moderates the indulgence of the sexual appetite. It is a form of the virtue of temperance, which controls according to right reason the desire for and use of those things which afford the greatest sensual pleasures. The sources of such delectation are food and drink, by means of which the life of the individual is conserved, and the union of the sexes, by means of which the permanence of the species is secured. Chastity, therefore, is allied to abstinence and sobriety; for, as by these latter the pleasures of the nutritive functions are rightly regulated, so by chastity the procreative appetite is duly restricted. Understood as interdicting all carnal pleasures, chastity is taken generally to be the same as continency, though between these two, Aristotle, as pointed out in the article on CONTINENCY, drew a marked distinction. With chastity is often confounded modesty, though this latter is properly but a special circumstance of chastity or rather, we might say, its complement. For modesty is the quality of delicate reserve and constraint with reference to all acts that give rise to shame, and is therefore the outpost and safeguard of chastity. It is hardly necessary to observe that the virtue under discussion may be a purely natural one. As such, its motive would be the natural decency seen in the control of the sexual appetite, according to the norm of reason. Such a motive springs from the dignity of human nature, which, without this rational sway, is degraded to brutish levels. But it is more particularly as a supernatural virtue that we would consider chastity. Viewed thus, its motives are discovered in the light of faith. These are particularly the words and example of Jesus Christ and the reverence that is owing to the human body as the temple of the Holy Ghost, as incorporated into that mystic body of which Christ is the head, as the recipient of the Blessed Eucharist, and finally, as destined to share hereafter with the soul a life of eternal glory. According as chastity would exclude all voluntary Carnal pleasures, or allow this gratification only within prescribed limits, it is known as absolute or relative. The former is enjoined upon the unmarried, the latter is incumbent upon those within the marriage state. The indulgence of the sexual appetite being prohibited to all outside of legitimate wedlock, the wilful impulse to it in the unmarried, like the wilful impulse to anything unlawful, is forbidden. Moreover, such is the intensity of the sexual passion that this impulse is perilously apt to bear away the will before it. Hence, when wilful, it is a grave offence of its very nature. It must be observed too, that this impulse is constituted, not merely by an effective desire, but by every voluntary impure thought. Besides the classification already given, there is another, according to which chastity is distinguished as perfect, or imperfect. The first-mentioned is the virtue of those who, in order to devote themselves more unreservedly to God and their spiritual interests, resolve to refrain perpetually from even the licit pleasures of the marital state. When this resolution is made by one who has never known the gratification allowed in marriage, perfect chastity becomes virginity. Because of these two elements — the high purpose and the absolute inexperience — just referred to, virginal chastity takes on the character of a special virtue distinct from that which connotes abstinence merely from illicit carnal pleasure. Nor is it necessary that the resolution implied in virginity be fortified by a vow, though as practised ordinarily and in the most perfect manner, virginal chastity, as St. Thomas following St. Augustine, would imply, supposes a vow. (Summa Theologiæ II-II.152.3 ad 4) The special virtue we are here considering involves a physical integrity. Yet while the Church demands this integrity in those who would wear the veil of consecrated virgins, it is but an accidental quality and may be lost without detriment to that higher spiritual integrity in which formally the virtue of virginity resides. The latter integrity is necessary and is alone sufficient to win the aureole said to await virgins as a special heavenly reward (St. Thomas, Suppl., Q. xcvi, a. 5). Imperfect chastity is that which is proper to the state of those who have not as yet entered wedlock without however having renounced the intention of doing so, of those also who are joined by the bonds of legitimate marriage, and finally of those who have outlived their marital partners. However in the case of those last mentioned the resolution may be taken which obviously would make the chastity practised that which we have defined as the perfect kind.