A man is an adult male human. He is the opposite of a woman. People use the word "man" (one man, two or more men) to talk about gender. Manhood is the period in a male's life after he changes from a boy. A boy is a male child; a female child is called a girl. After boys reach maturity, they can be called a man.
Like most other male mammals, a man inherits an X chromosome from his mother and a Y chromosome from his father. The men's rights movement is a group that tries to fight for men's rights and opposes misandry.
There are also those who identify as a man even though they are not one (they are not a member of the male sex). They are referred to as a man in certain societies.