A therapist is an umbrella term which can include many different types of mental health professionals including social workers, family therapists, clinical counselors, psychologists, and can even extend to occupational therapists, physical therapists, speech therapists, etc.
A psychologist is a subset of therapists and signifies they are a Licensed Psychologist in their state. Just like other licenses, a Psychologist refers to a specific license with specific training and criteria to obtain.