What is tissue? Well, there are several types of tissue within the human body. Epithelial tissues are those that cover organs, and the most predominant of these tissues is, of course, the skin. Then you have smooth muscle tissue and skeletal muscle tissue. The smooth tissues are found mostly internally in the esophagus, bladder, and other areas. It's the skeletal tissues that are the target of most forms of massage as they are the ones that typically cause soreness and discomfort.
Of the many types of massage, deep tissue work is one of the most beneficial. This form of massage typically focuses only on specific areas and it can be rather intense. There is usually soreness for a day or two following a session, but if done right, the benefits will be worth it.