Brahmacarya, or celibate student life, is the first spiritual stage (asrama) in the varnasrama system. It lasts from 5th to 25th year of of one's life and during this time the brahmacari lives with his spiritual master and learns the theory and practice of the Vedic life recorded in the revealed scriptures. 
After this time he is allowed to marry and enter the stage of householder life (grhastha asrama). Some brahmacaris, however, remain celibate the whole life.
Brahmacarya is defined as an abstention from the sexual gratification on the level of body, words and mind. This rule applies also to vanaprasthas and sannyasis.
There are 4 types of brahmacaris: (Krishna, the Supreme Personality of Godhead, ch.87, Prayers by the Personified Vedas, pp 407-408)
1. Savitra: After initiation and the sacred thread ceremony must observe at least three days of celibacy.
2. Prajapatya: Strictly observes celibacy for at least one year after initiation.
3. Brahma-brahmacari: Observes celibacy from the time of initiation up to the time of the completion of his study of the Vedic literature.
4. Naisthika: Remains celibate throughout his whole life. 
The first 3 classes are called upakurvana, which means that the brahmacari can marry later, after the brahmacari period is over.
"There are two-fold conditions in all asramas. Brahmacari becomes upakurvana or naisthika. Naisthika is eager to realize Brahman. "After studying the Vedas if he becomes householder he is called upakurvana. The naisthika remains brahmacari till death." (Garuda Purana 1.49.6-7)