Interpretation is rendered in two modes: simultaneous and consecutive. In simultaneous interpreting (SI), the interpreter immediately speaks the message in the target-language whilst listening to it in the source language. In consecutive interpreting (CI), the interpreter renders the source-language message after the source-language speaker pauses.

Consecutive interpretation rendered as ‘short CI’ and ‘long CI’. In short CI, the interpreter relies on memory; each message segment being brief enough to memorise. In long CI, the interpreter takes notes of the message to aid rendering long passages. These informal divisions are established with the client before the interpretation is effected, depending upon the subject, its complexity, and the purpose of the interpretation.

On occasion, document sight translation is required of the interpreter, usually in consecutive interpretation work. Sight translation combines interpretation and translation; the interpreter must read aloud the source-language document to the target-language as if it were written in the target language. Sight translation occurs usually, but not exclusively, in judicial and medical work.

Relay interpretation occurs when several languages are the target-language. A source-language interpreter renders the message to a language common to every interpreter, who then renders the message to his or her specific target-language. For example, a Japanese source message first is rendered to English, then it is rendered to the other target-languages.

Simultaneous interpreting

In simultaneous interpretation, the interpreter renders the message in the target-language as quickly as he or she can formulate it from the source language, while the source-language speaker continuously speaks; sitting in a sound-proof booth, the SI interpreter speaks into a microphone, while clearly seeing and hearing the source-language speaker via earphones. The simultaneous interpretation is rendered to the target-language listeners via their earphones. Moreover, SI is the common mode used by sign language interpreters. NOTE: Laymen often incorrectly describe SI and the SI interpreter as ‘simultaneous translation’ and as the ‘simultaneous translator’, ignoring the definite distinction between interpretation and translation.

Whispered interpreting

In whispered interpreting (chuchotage, in French), the interpreter sits or stands next to the small target-language audience whilst whispering a simultaneous interpretation of the matter to hand; this method requires no equipment. Chuchotage is used in circumstances where the majority of a group speaks the source language, and a minority (ideally no more than three persons) do not speak it.

Consecutive interpreting
In consecutive interpreting, the interpreter speaks after the source-language speaker has finished speaking. The speech is divided into segments, and CI interpreter sits or stands beside the source-language speaker, listening and taking notes as the speaker progresses through the message. When the speaker pauses or finishes speaking, the interpreter then renders the entire message in the target language.

Consecutively-interpreted speeches, or segments of them, tend to be short. Fifty years ago, the CI interpreter would render speeches of 20 or 30 minutes, today, 10 or 15 minutes is considered long. Often, the source-language speaker is unaware that he or she may speak at length before the CI interpretation is rendered, and might stop after each sentence to await its target-language rendering. Often, the interpreter asks the speaker to pause after each sentence; sentence-by-sentence interpreting requires less memorisation, yet, its disadvantage is in the interpreter’s not having heard the entire speech or its gist, therefore making more difficult establishing the accurate register by not knowing the exact vocabulary and terms to use. This method is usual in rendering speeches, depositions, recorded statements, court witness testimony, and medical and job interviews.

Consecutive interpreting allows for the source-language message’s full meaning to be understood before the interpreter renders it to the target language. This affords a truer, accurate, and accessible interpretation than does simultaneous interpretation.

Liaison interpreting

Liaison interpreting involves relaying what is spoken to one, between two, or among many people. This can be done after a short speech, or consecutively, sentence-by-sentence, or as chuchotage (whispering); aside from note taken then, no equipment is used.

Source : http://en.wikipedia.org/wiki/Interpreting

Functionally, an interpreter is a person who converts a source language to a target language. The interpreter’s function is conveying every semantic element (tone and register) and every intention and feeling of the message that the source-language speaker is directing to the target-language listeners.

Source :http://en.wikipedia.org/wiki/Interpreting