An interpreter is someone who translates a language, as it is being spoken or signed, into another one.
A distinction is generally made between translation, where the source and target texts are written, from interpreting or interpretation, where the source and target are spoken.
In some interpreting situations, the translation is given as the source-text speaker is still speaking (simultaneous interpreting); in others, a paragraph-by-paragraph translation is given, with the source-text speaker taking alternate turns at the microphone (consecutive interpreting).
It is important to realise that, while any translation from one language to another can never be completely accurate (e.g. there is no translation into Swahili for the word 'lawyer', as such a profession does not exist in that linguistic group), the trained interpreter tries to interpret not only the spoken words as accurately as possible, but also takes into account the intent of the message (cultural translation).