A psychiatrist is a medical doctor by training, and has obtained post-graduate qualifications and training to specialise in the field of Psychiatry. A psychiatrist is able to diagnose clinical psychiatric conditions,  prescribe medication treatment, and may also provide other forms of treatment such as neurostimulation.
A psychologist is a professional trained in understanding the emotions, cognitive processes and behaviour patterns of people. They may perform psychological testing, or provide psychotherapy as a treatment option for psychiatric disorders. Psychotherapy is also known as "talking therapy", and the approach is determined by the need of the client.
A counsellor is someone who is trained to provide general guidance and advice to people who need it. A counsellor differs from a psychologist, as the latter is trained in more specific therapy approaches such as CBT (cognitive behavioural therapy) to address psychological issues at a deeper level.