An interpreter works with spoken words in a particular context, conveying a message from one language to another, while translation refers to the activity of transferring a written text from one language to another.
Neither is simply replacing the words of one language by those of another, and there are similarities in the intellectual effort required. But there are significant differences between interpreting and translating.
The spoken word
Interpretation is spoken, translation is written. Interpretation therefore makes use of particular linguistic resources: the original speaker's ideas are transmitted as spoken words, with a particular rhythm and intonation, making use of rhetorical devices and gestures.
Interpretation is carried out in real time (simultaneously) or very close to it (consecutively). The interpreter has no time to refer to the written resources available to translators. This makes preparation before each assignment all the more essential for an interpreter.
Another constraint is the extreme speed at which the interpreter has to receive, understand, manage, and reconstruct information. A translator may translate 2000-3000 words a day, while an interpreter has to keep up with around 150 words a minute.
The context of communication
In interpretation communication is immediate, involving an interaction between speakers, listeners, and interpreters. In translation there is always a gap between the writing of a text by an author and its reception by the readers.
Apart from this, translators often spend a long time working on one text, while interpreters, often working in a team, are faced with people speaking and communicating right now.