Relaxation is a common by-product of meditation. Relaxation itself can assume many forms, such as taking a hot bath or reclining in the Lazy-boy and watching tv, etc. Meditation is an active process where the meditator remains fully aware of what the awareness is doing. It also attempts to transcend the thought process whereas many forms of relaxation still engage the thought process. Meditation allows the body to relax and can offset the effects of stress both mentally and physically to a potentially much greater degree than passive relaxation.
Thoughts generally consume energy in the process of their formation. Constant thought-activity, especially of random nature, can tire the mind and even bring on headache. Meditation attempts to transcend this crude level of thought activity. Through regular practice one becomes aware that they are not their thoughts but that there is an awareness that exists independent of thought. Descartes (“I think, therefore I am”) obviously was not a regular meditator!
Meditation begins with concentration, but after an initial period of concentration, thought activity decreases and keeping the awareness focused becomes more spontaneous. At this point the person may or may not continue to employ the object of concentration.
Self-hypnosis, like meditation, involves at least an initial period of concentration on an object. However in hypnosis one does not try to maintain an awareness of the here-and-now, or to stay conscious of the process. Instead one essentially enters a sort of semi-conscious trance.