They serve God, who is Love, and the only agenda they know is the divine plan. Remember that angels are always with us, but that they only manifest themselves to us when God wills. We can't force it, nor should we try. The more we live our lives for God, in love and joy, in light and peace, and in service to others, the closer our angels will be to us and the more we will sense their presence and loving help to enable us to heal our lives. People have many different definitions for angels.
Some people consider angels to be any disincarnate being.
Other people refer to angels as the higher self. An interesting definition for angels comes from Alice Bailey. In her material, which is the foundation of all metaphysical knowledge today, she speaks of a parallel universe to our own. The elementals take form in that Universe in what the Greeks call the Silfs of the air, the Gnomes of the earth, the Salamanders of the fire and the Undine of the water. These beings do not go through a reincarnation cycle, rather they evolve through an unfoldment process.