An individual who, by written or Court appointment or by the effect of a statute, is given custody of both or either of the property or the person of one who is unable to manage their own affairs, such as a child or a mentally-disabled person.
American practise is to use the full term "legal guardian" whereas other common law jurisdictions use either guardian or custodians.
Most jurisdictions allow parents to specify, in a will or will-like document, a third-party to become the legal guardian of their children should both parents die before the child(ren) attain the age of majority, subject always to the jurisdiction of the Court to vary the designation if it is appropriate.
A legal guardian exercises most of the legal rights of the person for whom they are legal guardian, until they either reach the age of majority or, in the case of legal guardianship of an incompetent adult, otherwise retrieve their capacity to manage their own affairs.