Generally a translator translates written documents from one language to another although sometimes he/she can also translate spoken words. An interpreter specifically translates spoken words as they are spoken. The people who sit in the booths at international meetings and provide simultaneous translation over headphones of whatever each speaker is saying into the language of other delegates is an interpreter. A person who sits in an office and translates a technical manual from one language to another is a translator. Interpreting is quite a stressful job and usually they do it in brief periods so many interpreters are needed to cover a long meeting.
PS Depending on their skills the same person might do both jobs.