A minor is someone under the age of consent, that is, a child in the eyes of the law. Sex with a minor is often illegal, and is certainly frowned upon, since it is generally considered that they cannot give informed consent to this sort of activity. In some jurisdictions, a minor is also the younger person in a significant relationship with a sufficient age gap, or the dependant in a relationship, especially when still quite young (e.g. under 21) even though they may otherwise be over the age of consent.
The age of majority in the UK is 18; however, the age of consent for sex (both heterosexual and homosexual) is 16 for most acts, although some restrictions apply. In the United States, minority varies with state of jurisdiction, and is much more complex. In addition, there are some Federal definitions of minority for such issues as interstate transportation.
As in all legal matters, please do not rely on Wipipedia but consult a legal profesional.