When people first hear the word psychotherapy, it often reminds them of psychiatry, however the two are not that similar at all. Psychotherapy is much more similar to counselling in that it is a talking based therapy designed to help you gain new insights and awareness's into your problems.
Many people confuse the two treatments, however there is a difference between counselling and psychotherapy. As such, when you see a therapist, you should ensure they are appropriately trained and qualified for the type of treatment you are looking for.
In general, counselling is a shorter term treatment (e.g. several months) and often involves dealing with the thinking portion of the mind to help the client overcome a problem in the here and now. Examples of this may be a dilemma involving work, exams, family or friends. It may be developing coping mechanisms for dealing with moderate stress levels or it can be used to overcome habits and compulsions. A common counselling method is CBT (Cognitive Behavioural Therapy) which has become more and more popular in recent years and which is most often used in the NHS.
Psychotherapy tends to be more of a longer term treatment which can sometimes carry on for several years. Clients who come for psychotherapy often have issues that they have struggled with for a long time or issues that reoccur throughout their life. The treatment focuses more on the feeling part of the brain, the emotional side of you, to help unlock feelings, thoughts and desires which you may not have consciously been aware of before or that you have been unable to express in the past. Examples of psychotherapy issues could be struggling to form and maintain healthy relationships, abuse of food, alcohol, drugs or other substances, depression, anxiety, excessive stress, self-destructive behaviours and many others.
As psychotherapists are dealing with much deeper, raw emotional states, they have to have more training than a counsellor. Typically, a counselling qualification takes a minimum of 3 years where as a psychotherapy qualification takes a minimum of 4 years.
If you are interested in any psychotherapy / counselling training yourself, a good place to find out more information is through the Metanoia Institute: http://www.metanoia.ac.uk/