Interpreting means rendering the content of a speech in one language into another language, as opposed to translation, which deals only with the written word. Technically, there are two basic kinds of interpreting: consecutive and simultaneous interpreting.
Consecutive interpreting
In consecutive interpreting, first the original speaker delivers his or her speech in the source language and then the interpreter repeats it in the target language. It can be done either in small segments (such as a sentence or a semantic unit) or larger (not more than several minutes), during which the interpreter takes notes. The greatest advantage of consecutive interpreting is that there is no need for special equipment: a pencil and a simple notepad are all the interpreter needs. However, there are many occasions for which this form of interpreting is unsuitable; it effectively doubles the time necessary for every single speaker, which may be rather tiresome for the listeners, especially those who speak both languages.
Simultaneous interpreting
In simultaneous interpreting, the de facto standard at multi-language conferences, the speaker and the interpreter speak at the same time: the speaker on the stage into his or her microphone, the interpreters in a booth (from which they can see the speaker and the audience) into their own. People in the audience who want to listen to the interpreting have to wear headphones; the sound is virtually always transmitted wirelessly. With the right equipment (many electronics manufacturers offer their own highly sophisticated systems), it is possible to have several teams interpret into many languages at once. The record holders in this field are the institutions of the European Union, able to organise the interpreting of the same speech into the languages of all member states (because of the sheer number of interpreters needed for this task, not to mention the extremely complicated logistics, this is not done for every session).
The disadvantage of simultaneous interpreting is its dependence on specialised equipment. To provide this service, an interpreting system has to be set up (there are many companies who offer the rental of a complete set together with the services of a sound technician), and two interpreters for every language who take turns in front of the microphone. This is absolutely essential, as interpreting is a very demanding activity that cannot be done for longer periods than about 15–20 minutes at a time, after which the quality of the output rapidly declines (the equally long break that you get while the other interpreter speaks is usually enough to renew your strength). The interpreter who isn’t currently speaking isn’t just sitting idle: it’s their job to help their colleague for example by writing down numbers, names, looking up information in various papers and everything else the speaking interpreter doesn’t have time for.
A special kind of simultaneous interpreting is chuchotage, whispered interpreting. There is no equipment necessary for chuchotage, but it can only be used in a very limited set of circumstances, with only a handful of people listening to the interpreter (in a group of more than three people, it is virtually impossible to ensure that everyone hears properly; the interpreters may not raise their voice, as it would disturb the speaker and the audience). Chuchotage is always somewhat clumsy for everyone involved, but in some cases it may be the best solution. Chuchotage can be also done with technical equipment, becoming basically a form of simultaneous interpreting without booths.
Preparation
Interpreters are not (and cannot be) experts in everything. Nevertheless, they can interpret even highly technical conferences that a layman would find almost impossible to understand if they prepare thoroughly. In general, the better materials and more detailed information the interpreters receive in advance from the client, the better the interpreting. In the case of conferences, this means for example PowerPoint presentations, ideally the paper that will be presented (even a version that is not quite finished is much better than nothing at all), or at least an abstract. Interpreters who did not receive anything in advance and knew only the title of a paper may easily find themselves in a situation where the speaker suddenly starts using terms and concepts they have never encountered before; the quality of their interpreting then unavoidably suffers. Giving the interpreters the opportunity to study up on the topic well in advance can avoid many potential problems.
No interpreting is perfect; however, if the communication between the interpreter and the client works well, no task is impossible.