The main difference between normal massage and erotic massage is the areas of focus. The massage therapist works on different parts of the body which trigger sexual energies. One important point to understand here is that most erotic massage businesses are not a cover for shady practices.
The sexual energy is the elixir of life. It is usually retained in the cultural context. Through massage the energy comes to life and flows through your whole body. Erotic massages are sometimes performed for pleasure but can also help treat various sexual health problems the client may be suffering from. This is a legitimate form of massage.