Shiatsu has roots in Daoism, traditional massage techniques of Japan (Anma) and traditional Chinese medicine, connecting mind/emotions and body together and to the energetics of the wider environment.
Shiatsu is a clothed body massage which involves the use of relaxed body pressure, mainly in the form of palming and thumbing, but also with vibrational, tapping and other techniques. It follows along specific energetic pathways of the body (as per acupuncture) (which to a western mind might equate with the connections of the nervous system and the fascia of the body.)
I practice 'Zen' shiatsu which involves a process of tonifying and dispersing techniques, according to how the energetic system of the Receiver presents itself through diagnosis of energy lines on the 'Hara' (abdominal centre) at a given time. Shiatsu connects deeply into the body/mind/emotional centre.
For a shiatsu session you wear comfortable trousers/top and socks, and cloths are used on areas of bare skin. You might lie on your back/sides/stomach or a combination of all of these for a shiatsu massage. Shiatsu can also be carried out on someone sitting or standing - but I don't normally do this.
Shiatsu is performed traditionally on a futon or more recently, using a massage bench - I use a massage table.
Before a session it's best to avoid a heavy meal and its also a good idea to allow a stretch of time to be quiet in after a session in order for the experience to sink in.