Tender points Trigger points and Pressure points
Tender points, Trigger points and Pressure points (usually called acupressure points) are all different, but they are often mixed up, even by doctors.
Tender points (TPs)
Tender points, not trigger points, are part of the American College of Rheumatology's criteria for the diagnosis of Fibromyalgia Syndrome (Fibro).
The anatomic and physiological mechanisms accounting for the presence of TPs have received great attention, but nothing special about TPs has been found in research. Many experts now agree that they are just points on the body where everyone is more sensitive and so where it is easier to note and in some way quantify the hypersensitivity to pain of Fibro patients.
TPs occur in pairs on various parts of the body . TPs hurt where pressed, but do not refer pain elsewhere and they are not hard knots (unlike Trigger Points) - in fact there is nothing to be found at a TP site other than tenderness.
The American College of Rheumatology's guidelines for the diagnosis of Fibromyalgia  is that the patient should have widespread pain in all four quadrants of the body for a minimum duration of three months and tenderness or pain in at least 11 of the 18 specified tender points when a specific amount of pressure is applied (see figure below). Most healthy individuals experience pain in only a small number of tender points in response to this test. 
However, the criteria of “11 of 18" specific Fibro tender points were originally meant to screen patients for clinical study and not as diagnosis . A range of things affect TPs including illness progression, injuries  and also gender - men with Fibro tend to "score" lower than women with Fibro, which is one factor behind the under-diagnosis of Fibro in men. For the tender point exam to have any meaning, you need a doctor who really knows what they are doing. See the article Explaining thr Tender Point Test for more details. A key part of this is knowing the difference between Tender Points and Trigger Points and being able to differentiate between them in the examination.
(Myofascial) Trigger points (TrPs or MTPs)
Trigger points are hard points in the myofascia that hurt to the touch and refer pain and/or other symptoms elsewhere.
There is no such thing as a Fibromyalgia Syndrome trigger point.
Fascia is the semiflexible fibrous membrane of connective tissue that binds together the various components of the body.  The Myofascia is the fascia relating to the muscles - it covers individual muscle fibers, bundles them together and covers the whole muscles .
A substance within the myofascia is involved with the transfer of nutrients and removal of waste products in the muscles, and also keeps the muscle supple, preventing microadhesions from forming. If the myofascia is subjected to biochemical or mechanical trauma, then the fluids in it can thicken, even becoming hard, making the myofascia tighten. Microadhesions then form between the muscle fibres, which is what the tight bands or knots of the TrPs are. 
This fibrous myofascial adhesion affects the nerves around the muscle, disrupting their normal function, which is how the TrPs trigger symptoms away from themselves. 
TrPs seem to form as a response to things happening to our bodies, such as overuse, repetitive motion trauma, bruises, strains, joint problems, surgery, or stress. Pain creates a neuromuscular response, and the muscle around the pain site tightens, "guarding" the hurt area. Stress can cause you to tense muscles for no reason other than a stilled form of the fight or flight instinct. 
When muscles are in a state of sustained tension, they are working, even if you're not. A working muscle needs more nutrition and oxygen, and produces more waste, than a muscle at rest. And unlike when you exercise and work muscles that way, increased blood flow isn't supplied to deal with the demands. This creates an area in the myofascia starved for food and oxygen and loaded with toxic waste — a TrP. 
An active TrP not only hurts when it is pressed, but it "triggers" a referred pain pattern or other symptoms locally or elsewhere in the body. This pattern from specific TrPs is usually similar from patient to patient and charts are available depicting pain referral patterns from TrPs. An active TrP hurts whenever you use the involved muscle and if a TrP becomes very active, symptoms occur even when the muscle is at rest.
TrPs can cause a wide range of symptoms including:
- Stabbing pains.
- Burning pain.
- Stuffy sinuses.
- Headaches & migraine.
- Reduced mobility.
Acupressure (a combination of "acupuncture" and "pressure") is a traditional Chinese medicine technique based on the same ideas as acupuncture. Acupressure involves placing physical pressure by hand, elbow, or with the aid of various devices on different acupuncture points on the surface of the body. Acupressure points can do a variety of things including relieve pain.
- Starlanyl DJ, 2003. Fibromyalgia and Chronic Myofascial Pain: Keys to Diagnosis and Treatment [website]. Available: http://www.sover.net/~devstar/physinfo.htm [Nov 2008]
- Starlanyl DJ, 2004. Trigger Points and Tender Points: Why the Difference Is Important to You [online paper]. Available: http://www.sover.net/~devstar/TrPs_and_TPs.pdf [Nov 2008]
- Wolfe F, Smythe HA, Yunus MB, Bennett RM, Bombardier C, Goldenberg DL, et al. The American College of Rheumatology 1990 criteria for the classification of fibromyalgia: report of the multicenter criteria committee. Arthritis Rheum 1990;33:160---72.
- National Fibromyalgia Association. Glossary of Research and Other Fibromyalgia Terms and Definitions [website]. Available: http://www.fmaware.org/site/PageServer?pagename=resources_glossaryTermsDefinitions#T [Nov 2008]
- Clark GA. Fascia & Myofascia [website]. Available: http://www.painbustersclinic.com.au/body/fascia.htm [Nov 2008]
- Starlanyl DJ, Copeland ME, 2007. Fibromyalgia and Chronic Myofascial Pain: Patient Information [website]. Available: http://www.sover.net/~devstar/define.htm [Nov 2008]