This answer reflects the situation in the United States.
A therapist is a licensed counselor. They help clients treat mental health symptoms and work through managing their stress, relationships, daily and lifelong mental health issues, processing, and more.
A psychiatrist is a medical doctor who can diagnose and prescribe medication for mental health disorders.
Oftentimes, patients will work with both a therapist and a psychiatrist on a team (they’ll meet separately generally, but there should be communication between the two) – with their therapist meetings happening on a more regular basis, while they will meet with their psychiatrist more infrequently for medication check-ins and prescriptions, diagnosis meetings, and more.