Actually very little but loosely speaking the term of treatment and the depth of the counselor's (or psychotherapist's) training.
In respect of training, accreditation organisations in the industry loosely distinguish counselors from psychotherapist by the depth and duration of the training. A psychotherapist will generally hold a post graduate diploma or masters level (or higher) qualification (greater than 2 years of full time training) and a counselor's training will be less extensive and shorter (for example undergraduate diploma level or less than two years of full time training). In law there is nothing to prevent a practitioner from using the terms counselor and psychotherapist interchangeably however the industry generally holds that a psychotherapist may offer counseling or psychotherapy whereas a counselor lacks the training to offer psychotherapy.
In respect of treatment, the industry generally considers psychotherapy to be longer term (anywhere from 6 months to decades) of in depth work with a view to major personal growth. By contrast, counseling is considered to focus more on specific issues and be brief (5 to 10 sessions).
In practice, I think these definitions are misleading to the public. I think for example that someone trained in counseling may still offer effective in depth long term work after they have gained sufficient experience and if they have a natural ability to be intuitive and empathic. I also think it is possible for clients to benefit from short term work in a psychotherapeutic way. Further, to the client, what actually goes on any given counseling or psychotherapy session would seem (and most likely actually be) indistinguishable one from the other. There is also considerable research that suggests it is not what counselors, psychotherapists (and coaches for that matter) do but simply the quality of relationship they achieve with their clients and how they facilitate the client's change through that relationship.