Guardianships are used in two situations.
The person upon whom the guardianship is imposed is called the ward.
Guardianships for children and adults are similar. They are imposed to protect the ward both physically (guardianship of the person) and financially (guardianship of the estate). A guardianship may be of the person, estate, or both.
More information on guardianships for children can be found here and more information on guardianships for adults can be found here.