Deep tissue massage is simply any massage aimed at the deeper layers of muscle and connective tissue in the body.
While some of the postings above touched on good points of technique for applying deep tissue massage, the techniques themselves are not the defining factor of what is and isnt deep tissue massage.
Case in point; Craniosacral therapy affects the deepest layers of connective tissue in the body, and therefore could be considered the deepest form of massage therapy available. While it generally carries none of the hallmarks of what is "traditionally" considered a deep tissue massage, an argument could be (and has been) made for its inclusion under the above definition.
When my clients ask me to define deep tissue massage (as it is the way I describe the majority of my practice) I often take the opportunity to explain that the definition (as stated above) is particularly vague and that a better question is to ask each individual practitioner they are considering working with to describe their training and what a typical session with them is like.