An interpreter is a person who receives a non-recorded message spoken in one language, the source language, and repeats the same message in a different language, the target language.
Unlike a translator, an interpreter’s source is volatile:
He cannot simply rewind a recording and hear the same speech again.
This makes an interpreter’s job difficult.
He must remember many parts of speech.

Interpreters help to communicate between people speaking different languages.
Some people do not understand fast speech in the source language and find an interpreter speaking their first language useful, too.
In rare instances, an “interpreter” could also speak a speech in the same language as the speaker, because the speaker’s accent is too difficult to understand for other people.

Technique

Modes 
Many interpreters perform simultaneous interpretation (SI).
In this mode of interpretation the interpreter produces the message in the target language at the same time.
There is a certain delay called  before an interpreter starts speaking.
Depending on the source language’s difficulty, this may be 2 to 16 seconds.
This mode is preferred if listeners possibly will reply after a short time.
It is used, for example, in the United Nations General Assembly.

Some interpreters also offer consecutive interpretation.
Here, the interpreter listens to a speech for 10 to 15 minutes and takes notes.
After that, the interpreter reproduces the speech in the target language, taking his notes for help.
This mode is sometimes used at conferences, when the audience primarily speaks the target language, but the speaker is an exception.
It does not need headphones for listeners and is more pleasant, because there is no concurrent mix of different voices.

Modality 

There are interpreters acting between modalities:
An interpreter can listen with his ears and sign the speech for deaf people.
There are also interpreters who speak with their mouth while seeing a (possibly deaf) person signing.
There are barely any interpreters directly interpreting between sign languages.

Performance 
Because interpretation is a very difficult task due to the time requirements, interpreters focus on transmitting the main information of a message.
They do not reproduce hesitating discourse markers, like “uhm”, “err”, “hmm”.
Words that only embellish speech are omitted.
Difficult sentence constructions is simplified.

On the other hand, interpreters try their best to transmit ideas across cultures.
Proverbs, idioms and figures of speech are mapped to their closest equivalent in the target language, taking account of given context and the audience’s culture.
If applicable, interpreters try to preserve the same register, formal or informal language.

Career

Training 
In order to become a successful interpreter, a candidate must have perfect command of both languages.
Often it is further helpful to have good knowledge of the subject matter.
The job is to transfer ideas.
It is difficult to interpret an idea that is not understood by the interpreter.
For that reason, an interpreter, for instance, specialized in court hearings does not only need to know legal terms and their respective translations, but also what they mean and how they correlate to each other.
It is not necessary to become a lawyer, though.

Labor market 
Most interpreters are self-employed.
Even the European Parliament employs permanently only a small subset of needed interpreters, namely those who are also translating documents if there is no parliament meeting.
Interpreters generally also offer translation services.

Related pages 
 Video Relay Service
 Teletypewriter

Sources 

Linguistics