A psychologist is a scientist who studies the mind. Unless specified, you should assume the psychologist studies the human mind — as opposed to, for example, a pet psychologist.
A psychologist can be a researcher, who makes abstract studies of mental disorders and their treatments. Or, a psychologist can be licensed to see and treat patients, offering therapy for mental conditions or behavioral issues. The word comes from two Latin root, psyche, meaning "soul, mind, and spirit," and logia, meaning "the study of." Unlike a psychiatrist, a psychologist does not have a medical degree, and therefore cannot prescribe medication.