“Deep tissue,” like most terms in the woolly world of massage therapy, has several meanings, none of them very precise.
1) It's mostly commonly used in contrast to “relaxation” massage, or “fluffy spa massage.” Deep tissue is serious massage, massage that really does things, that digs in and finds problems and fixes them. It's not just petting. In this sense, it's really just a vague word of praise: it carries the same positive force – and carries about the same information load – as “real.” That was a deep tissue massage; a real massage.
2) Its only reasonably precise meaning is also its most useless: it can mean “massage that reaches under the superficial muscles to reach tissues underneath.” All massage does this, of course; there's virtually no way to avoid it. And all competent massage therapists, however they advertise themselves, know how to move the body around so as to get better access to deeper muscles. A massage therapist who won't do this, who just pets an immobile body with a uniform light pressure, won't last a week. Even the supposedly fluffy relaxation therapists do this. Some are more aggressive than others, but they all do it.
3) There are a variety of methods that advertise themselves as “deep tissue,” some patented and some not. These are particularly common among people who specialize in, and talk a lot about, fascia (connective tissue) and adhesions. They can get very excited about the thixotropic properties of fascia – how it softens as it is warmed and worked – and about how it can get stuck to things. Some of them talk a very persuasive line. Unfortunately when it comes to results, no one has been able to show that they are better at fixing anything than the supposedly fluffy relaxation therapists. (See my review of Cherkin's low back pain study, which directly pitted therapists trained in “deep tissue” against therapists who described themselves as doing relaxation massage. The relaxation therapists got slightly better results.) As for adhesions – as Paul Ingraham writes, “In active people . . . it is basically impossible to develop any significant adhesions, anywhere in the body.” People would love to believe that their bodily problems are simple mechanical ones, which can be fixed in the same way you'd fix a car – bang on this part, lube that one up, take this conglomeration apart and reassemble it, and voila, now your neck works fine! But attractive as this idea is, it's almost never that simple. The fact is that the verifiable effects of massage are rather mysterious and maddeningly unspecific. Massage makes you feel better, sometimes dramatically better, but it's very hard to say why.
Now having said this much, I have to make an embarrassing confession. I have, on occasion, advertised myself as a deep tissue massage therapist. I did this for a very simple reason. I myself like firm massage. I like a therapist who is not squeamish, and who will use enough pressure to really make a difference, when it's called for. I don't like to get manhandled or beat up, but I don't mind if there's occasional pain along the way, so long as I'm confident that my therapist knows what she's doing and won't really hurt me. Long before I trained as a massage therapist, as a client, I searched for therapists who would do that sort of massage, and I vaguely associated the term “deep tissue” with it. It was to tell my prospective clients that I could do that, if they wanted it, that I called myself skilled in “deep tissue” massage. (I have in fact studied fascial methods, pin and stretch techniques, various contract-and-relax protocols, so I wasn't just whistling Dixie. But I don't have a whole lot of faith in any of that stuff, as practiced by massage therapists.)
And the point here is that massage therapists can basically say whatever they want. It's very much a caveat emptor world: let the buyer beware. Someone who bills herself as a “deep tissue therapist” is probably one who isn't afraid to use a lot of pressure, which can be a good thing or a bad thing, depending on how sensitive her hands are and good she is at reading the body. But it doesn't tell you much more than that.