This is a period of formal education. It lasts until the age of 25, during which, the young male leaves home to stay with a guru and attain both spiritual and practical knowledge. During this period, he is called a brahmachari, and is prepared for his future profession, as well as for his family, and social and religious life ahead.
This period begins when a man gets married, and undertakes the responsibility for earning a living and supporting his family. At this stage, Hinduism supports the pursuit of wealth (artha) as a necessity, and indulgence in sexual pleasure (kama), under certain defined social and cosmic norms. This ashrama lasts until around the age of 50. According to the Laws of Manu, when a person's skin wrinkles and his hair greys, he should go out into the forest. However, in real life, most Hindus are so much in love with this second ashrama that the Grihastha stage lasts a lifetime!
This stage of a man begins when his duty as a householder comes to an end: He has become a grandfather, his children are grown up, and have established lives of their own. At this age, he should renounce all physical, material and sexual pleasures, retire from his social and professional life, leave his home, and go to live in a forest hut, spending his time in prayers. He is allowed to take his wife along, but is supposed to maintain little contact with the family. This kind of life is indeed very harsh and cruel for an aged person. No wonder, this third ashrama is now nearly obsolete.
At this stage, a man is supposed to be totally devoted to God. He is a sannyasi, he has no home, no other attachment; he has renounced all desires, fears and hopes, duties and responsibilities. He is virtually merged with God, all his worldly ties are broken, and his sole concern becomes attaining moksha, or release from the circle of birth and death. (Suffice it to say, very few Hindu men can go up to this stage of becoming a complete ascetic.) When he dies, the funeral ceremonies (Pretakarma) are performed by his son and heir.
Although these ashramas are predominantly designed for the male, females too have a vital role to play in each one of them. So women are not actually excluded because they are always supposed to have an active social and religious life at home. However, a woman's role is of a dependent nature since, traditionally, they need the protection of a responsible male at every stage of life.