What are these taut bands of muscles that hurt when I press them?

The most commonly referred definition of Trigger Points is one from a 1983 study of myofascial pain done by Travell and Simons. According to them, a TrP is "a hyperirritable spot, usually within a taut band of skeletal muscle or in the muscles fascia. The spot is painful on compression and can give rise to characteristic referred pain, tenderness and autonomic phenomena." Trigger points can come about as the result of an acute trauma such as an accident or injury, but they can also be chronic, coming as the result of small, repeated activities that put stress on an particular area of the body.