What is a trigger point?  The following description (which is very good!) is taken from Clair Davies' book, "The Trigger Point Therapy Workbook," first edition p. 3.

Travell and Simons describe a trigger point as simply a contraction knot in the muscle tissue. It often feels like a partly cooked piece of macaroni, or like a pea buried deep in the muscle. A trigger point affects a muscle by keeping it both tight and weak. At the same time, a trigger point maintains a hard contraction on the muscle fibers that are directl connected to it. In turn, these taught bands of muscle fier keep constant tension on the muscle's attachments, often producing sympotms in adjacent joints. The constant tension in hte fibers of the trigger point itself restricts circulation in its immediate area. The resulting accumulation of the by-products of metabloism, as well as deprivation of the oxygen and nutrients needed for metabolism, can perpetuate trigger points for months or even years unless some intervention occurs. It's this self-sustaining vicious cycle that needs to be broken (Travell and Simons 1999: 71-75).

They often refer pain to other areas of the body. For instance, what you may think is a sinus headache can be caused by trigger points in the scalenes. Usually, therapists will work on trigger points with their thumbs, possibly fingers or elbows. Skilled Ashiatsu therapists can even use their feet, either the heels or the big toe, to release trigger points.

For those that self-treat, a variety of tools can be used. I frequently recommend a tennis ball or super ball on clients' upper back, hips, and those often problematic QLs (Quadratus Lumborums). Other people use devices like a Theracane, BackKnobber, Knobble or T-bar. In an emergency, a well placed door knob can help reduce lower back and hip pain!