E-therapy is when a professional counsellor or psychotherapist corresponds with you over the web, to give you emotional support, mental health advice or some other professional service. It could be one question, or an ongoing conversation; it could be by email, chat, video or even internet phone (e.g. Skpe).
E-therapy is a viable alternative source of help when traditional psychotherapy is not accessible. It's effective, private and conducted by skilled, qualified, ethical professionals. For some people, it's the only way they can get help from a professional therapist.
E-therapy is not psychotherapy, and should not be compared to it. It is not a substitute for traditional psychotherapy. Working with a therapist in person is still better. But many people cannot or will not see a therapist in person. E-therapy is a form of counselling which, though it falls short of full-fledged psychotherapy, can still be a very effective source of help.
For a directory of e-therapists, see http://www.metanoia.org/imhs/directry.htm, or do a Google search on "e-therapy".