Shi is finger, Atsu is pressure. Shiatsu massage is Japanese style massage. Therapy basic on the combination of the traditional Chinese theories.
Shiatsu is a type of massage that's based on pressure points. Theory behind it is similar to acupuncture, except with shiatsu there is no need for needles. The massage done with the fingers, though is some case the practitioner will use certain mechanical devices as and aid.
Somebody called this type of massage is acupressure. Actually shiatsu is different acupressure.