Acupressure is based upon the exact same traditional Chinese medicine ideas as acupuncture, but instead of using needles the practitioner applies physical pressure instead using either their fingers, palms of hands, knuckles or elbows as appropriate to those same acupuncture points on the body that an acupuncturist would stick a needle into. The effectiveness or effects of an acupuncture vs a acupressure session are generally identical in most cases and those who don't like needles would no doubt be able to consider this a viable alternative.
Many systems of traditional eastern medicine consider this common notion that there is a vital life force energy that flow like a kind of electricity throughout the body through various known channels or meridians as they are called in Chinese medicine. The flow of life force energy through any part of these channels within the body can become impeded or in excess, this is what is considered to be the determinant of health and well-being. The acupuncture or acupressure points are specific points along the channels which can act as gates to controlling the flow of life force energy when stimulated. Specific points are often associated as being helpful for particular ailments.
Acupressure massage which generally uses static pressure most of the time rather than rubbing or kneading can be particularly effective at relieving physical tension and emotional stress in the body as one can imagine, but as with acupuncture this type of therapy may be even more helpful with systemic type health challenges i.e. those which are less structurally related to the body in nature or where the originating centre is not readily identifiable.
Acupressure can be done with recipient sat in a chair or lying down either on the floor on top of a futon mattress or on a massage table. Shiatsu and Tui-Na are 2 common forms of oriental massage which use acupressure.
As it happens, here is recently filmed video example of yours truly doing some acupressure chair massage at a public event, it can be viewed here:
Hope this is helpful. Please feel free to ask for more info if needed.
Thanks and take care,