Lust exists in two forms: on the one hand, it is a sexual promiscuity and immoral behaviour that turn a person into an animal, and on the other hand, it is longing for carnal pleasures, their accumulation and absorption, feeding on them (not necessarily sexual ones). Lust is also a pleasant and satisfying feel, at first only mingling with our feelings, but then becoming an aim in itself.
The sin of lust is first born in thoughts, and there one must cut it short, not giving it a chance to increase. The lust of the eyes is a sudden flash of impure desire caused by a real, visible tempting object or its image. The lust of the flesh means prolonged sexual fantasies in the man's mind and sexual desires, which are only a step from sexual sins (adultery, prostitution, homosexuality, perverse sex …)
Lust is completely different from love: love respects and appreciates the beloved, wishing her (him) the best and being entirely focused on her (him). In love there is no selfishness, it is self-giving, tied with self-devotion; in contrast, lust seeks to use things or people only satisfying one's own needs and desires. The only concern of lust is lust itself; being selfish, it rejects devotion.
Chastity is innocence, strict morality, purity in body and soul. In a broad sense, chastity is synonymous with perfection: "Chastity is the full wisdom, soundness, and absence of damage, it is a normal condition of inner life, a personality's integrity and strength”, an ability to “preserve all one's virtues intact, keeping watch over oneself in all the actions, words, deeds, and thoughts.", – St. Amvrosii Optinsky
In a narrow sense, chastity is regarded as the spiritual and moral purity, the quality that is opposite to lust and the sexual arousal. "It is him that is truly chaste who saves not only his whole body from lust, but keeps his every member (the eyes, or the tongue) celibate, and in his inner man spiritual intents are not combined with sinuous thoughts." "Purity despises luxury, comfort, refined clothes. Purity hates expensive delicacies and avoids wines. Purity enslaves flesh and aspires for the holy.", – St. E. Sirin
Regardless of using the name of the virtue in a broad or narrow sense, the difference is made between its internal and external attributes. External ones are reflected in a person's behaviour, manner of clothing, and appearance. As for the internal chastity, it means doing good not for the people, but for God. The man who is chaste in his soul, does not consider himself complete, thus being able to keep his mind and heart from bad thoughts, desires and feelings.