Pat DeYoung - Relational Psychotherapy
That’s a hard question to answer -- partly because there are so many kinds of psychotherapy, from talk therapies to expressive arts therapies to cognitive/behavioural therapies.  Further, you can get help as a child, adolescent or adult, as an individual, couple or family, or as part of a group.  So any psychotherapist you choose will have been trained in only one (or a few) of the many forms of psychotherapy, and to work with only certain ages and configurations of clients.
But I’ll try to simplify:  In general, a psychotherapist is a professional who works with people who find themselves in certain kinds of trouble:  trouble with their inner world of emotions, thoughts, and moods, and often related trouble with their outer world of social activity, work, and family relationships.
A psychotherapist develops a formal or informal contract to work with a person, couple, or family on these kinds of life troubles, and enters into a helping relationship with them in which boundaries and mutual expectations are clear.  Usually that relationship continues as a series of regular meetings over a period of time, short-term or long-term.
But what’s a psychotherapist in relation to a psychologist or psychiatrist or social worker?  Don’t they all do that kind of work with people in emotional, psychological, and relational distress?
This isn’t simple to explain either, at least not now in Ontario.  Some psychotherapists have been educated and trained exclusively in psychotherapy, and that’s their entire and exclusive practice:  they call themselves psychotherapists. 
But some professionals who have been trained to practice psychotherapy have been educated in another profession as well -- family medicine, psychiatry, psychology, social work, nursing, occupational therapy, or pastoral care.  They call themselves psychotherapists if psychotherapy is the main work that they do, even though their credential is in another profession.  Or they may still call themselves, for example, a doctor, social worker, or psychologist, even while offering psychotherapy services. 
On April 1, 2015, the title Psychotherapist became protected under Ontario law; only those professionals who are well-trained in psychotherapy and registered with the College of Psychotherapists are allowed to call themselves a Psychotherapist  (further info at ).  Physicians, social workers, psychologists, OTs, and nurses will still be allowed to practice psychotherapy within their training and competence to do so.
What matters most for you as a psychotherapy client is that the person providing you psychotherapy services has been well-trained in psychotherapy, either exclusively or in addition to any other professional training.  For your protection, he or she should also belong to a college or professional association with a code of ethics, standards of practice, and complaints and discipline procedures.  Any psychotherapist should be willing to discuss his or her eduction, training, and professional associations with you.