Wikopedia describes psychotherapy as "An intentional interpersonal relationship used by trained psychotherapists to aid a client or patient in the problems of living. It aims to increase the individual's sense of their own wellbeing."
Counselling and psychotherapy use a similar range of skills. The therapist listens empathically and non-judgementally. She aims to create a safe space and relationship within which the client can share her/his thoughts and feelings.
The difference between counselling and psychotherapy is that the psychotherapist uses her knowledge and experience to understand the deeper psychological processes which the client is going through. She is also likely to have a deeper understanding of her own psychological processes (most psychotherapists - including myself have had extensive personal psychotherapy in order to understand themselves better). Psychotherapy can sometimes feel a bit more directive than counselling as the psychotherapist is likely to gently guide the client to a fuller understanding of his or her thoughts, feelings and behaviours. The insights gained can then help the client to change and to respond to situations differently. This can happen both at a conscious and unconscious level.
The relationship between the client and the therapist is extremely important if psychotherapy is to be effective so it is important for the client to feel comfortable to engage with the therapist. For this reason it is often a good idea to get a recommendation before going to see a therapist and to meet him/her before committing to working with them. I offer a short meeting free of charge for this purpose
Psychotherapy can be short term or long term depending on the degree of difficulty the client is experiencing and on the outcome the client wants.