The main differences for me personally are that counselling often tends to be shorter-term and may concentrate on specific, current or recent issues, whereas psychotherapy tends to be longer term, usually focusing on deeper, more complex issues or patterns rooted in the past, and which the person may not be aware of.
Psychotherapists have a longer training period and are required to have their own therapy throughout as part of their training. However, defining the difference is not an exact science, and in practice there is often much overlap between the two. A lot depends on the approach of the individual practitioner. You do not need to know in advance which you need – this is something we would discuss when we meet, and often it evolves naturally, according to your needs and situation.