The term "psychologist" implies several different occupations and is different to the term "psychiatrist".
Working psychologists are often classified as clinical, educational, or industrual depending on which field they work in. Clinical psychologists can be similar to psychiatrists without a medical diploma (and therefore without the authority to prescribe drugs etc). They are both concerned with psychopathology (abnormal behavior). Personality traits of such practitioners are important when dealing with abnormal behavior, and many pracitioners have experienced occasional personal "problems" with respect to their relationships with their clients.
Academic psychologists in higher education institutes teach and do research, and can adopt research specialisms such as psycholinguistics, social psychology, and cognitive psychology. (etc)