A minor is a person who is below the legal age of adulthood, the age of majority, which is currently typically 18, 19, 20 or 21, depending on which country and state you live in. In most places today, the age of majority is 18. Such a person is also said to be underage.
Note that the definition of a minor is in practically all countries differrent from that of a child. However, in some cases, the line between the terms "child" and "minor" is blurred, for example in the term child pornography.
Generally a contract with a minor may be legally void or voidable, unless the minor's parents or guardian has ratified it, but this depends on the jurisdiction involved and the nature of the contract. Minors may not be allowed to give consent to medical treatment, or to sexual relations. Sexual intercourse with a minor may be a crime, as may images of a minor having sex or in a sexual situation. Minors may not be legally allowed to purchase or possess erotica or pornography. Again, much depends on the particular jurisdiction involved.