https://tools.ietf.org/html/rfc6749#page-6
    resource owner
        an entity that is able to grant access to its protected resources
    authorization server
        grants access tokens to Clients after successfully authenticating
        Resource Owners and obtaining their authorization
    resource server
        a component that requires an access token to allow,
        or at least consider, access to its resources
    client
        an entity that is capable of obtaining access tokens from authorization servers