Someone whose job is intermediating in oral communication in situations involving speakers of different languages; to be clearly distinguished from translators, who work with written texts. There are considerable differences between the skill sets involved and the appropriate personality types for the jobs. Interpreters, or rather interpreting jobs, come in several different flavours:

Simultaneous interpreting
The most common popular image of the interpreter, listening to the speaker and speaking in a different language at the same time without interrupting the speaker's own flow. May either be an individual accompanying a particular speaker who is operating in a different language context, as sometimes seen on TV at major international events, but more commonly to be found in glass booths at multilingual meetings and conferences, with their versions being put out on headphones to attendees from the relevant language group. The ability to listen and talk at the same time is a somewhat peculiar one, and pretty uncommon; the work is stressful and early career burnout is not uncommon. Conference interpreters generally work in teams of two or three per language pair.
Consecutive interpreting
Interpreting on a more or less sentence by sentence basis, alternating with the original speaker. This is notionally easier, but means that the interpreter needs to "manage" the discussion, and obviously slows things down. This is appropriate for one-to-one meetings, interviews, etc. and includes ...
Liaison interpreting
Consecutive interpreting working both ways, so that the interpreter is the only person who is actually aware of what both side are saying. The norm for community interpreting (e.g. between civil authorities and members of a linguistic minority group) or for those accompanying explorers or journalists in exotic locations.
Chuchotage
Simultaneous one-way whispered interpreting for a single person at a meeting who may have trouble understanding what is going on.
Voiceover
Used mainly on television news and documentaries, usually with the original voice still faintly audible in the background (but usually too quietly to be intelligible, which can be quite annoying for people who understand the source language. Clearly where this is done on recorded material (as is usually the case) it can be transcribed, translated and read by a non-interpreter, but professionals do tend to do it when speed is of the essence.

Most interpreters are self-employed and work via agencies, although major international organisations like the EU and the UN do employ staff. The way into the profession (in major languages) is mainly via specialist degrees and postgraduate training, although both the US and British military intelligence services (among others, no doubt) train up serving personnel in interpreting techniques.