A translator deals with written text. They translate from one language to another. A translator is needed for documents like a Birth Certificate, Driver's License and other written documents.

An interpreter deals with the spoken word. They interpret what someone says into another language. An interpreter is needed for situations where you need help in understanding what someone is saying or to help someone else understand what you are saying.

Examples:

A translator is needed to translate the text of a newspaper from one language to another.

An interpreter is needed to interpret a radio show from one language to another.