A good question and the answer is similar to that of ‘What is Counselling?’. There is no one definitive definition and people often view the differences differently! The training is broadly similar (normally taking four years or over to qualify) and similar theory and approaches will be learned.
To put it simply counselling is normally time limited (anything between four and sixteen sessions) and generally has a sharp focus on what the client would like to achieve.
Psychotherapy often does not have a limit to the number of sessions and can be flexible in what the person wishes to achieve, but similar theories and approaches can be used by both.
F.Y.I. I trained and qualified as a psychotherapist, but I have done a lot of counselling too.