Deep tissue massage therapy is directed specifically to relieving tension in deeper muscles and unsticking stuck-together individual muscle fibers that interfere with muscle movement.
Deep tissue is a somewhat generic term, as there are many ways of doing deep massage. A common myth is that deep massage has to be painful. IT DOES NOT!
Skilled deep tissue is about working with your body to reach deeper levels, not plowing through tissue no matter what. Sometimes it can be uncomfortable or intense, depending on your body's condition, but a good massage should not cause you a lot of pain. In fact, deep tissue expert Art Riggs wrote in a 2005 article in Massage & Bodywork that "painful work is usually ineffective because the muscles contract against the pain, instead of lengthening and relaxing."
According to Riggs, the key to effective deep massage is slow strokes that have a specific intention and purpose and that are done with the full attention of the massage therapist. Riggs discusses how the therapist should choose strategies and strokes that are driven with intention and function. He also states "The biggest cause of pain -- or harsh touch -- is attempting to make things happen, rather than letting things happen."