Professional interpreters can perform in both the simultaneous and consecutive modes. Simultaneous is when you sit in a booth and listen to the speaker through her microphone and your headset and you interpret at the same time with a lag of a few seconds. Consecutive, on the other hand, involves listening to the speaker until she stops and then repeating in the target language what was said precisely in all its nuances, including emotions, register, facts, numbers, names and so on. To do this (unless the speech is only a couple sentences long) you must take accurate notes. Over the years professional interpreters have honed a method of note taking that is different from any other.
People often ask if it is like shorthand. No, it is not. Shorthand is based on the form, the words. A secretary can reproduce all the words of an utterance using shorthand without understanding the message. Since interpreters must not only record the meaning but also express it in another language, interpreters' notes are based on the meaning. The interpreter must take in the message in all its color and factual content, as fully as any good listener, take accurate notes that will serve as reminders, and then give the speech again using his own words in the target language.
Such notes must be absolutely complete and clear. The consecutive interpreting note-taking method is based on keywords, initials, symbols and other graphics that together form a whole. Their placement in specific areas of the page further enriches and clarifies their meaning so that all necessary information can be recorded with minimal writing. This is crucial because the speed at which most speakers go strains one's ability to jot it all down and keep up.
It is an art. The method has been honed over decades by interpretation masters who perform the task and often teach in schools across the world. Learning to use the technique is much like learning a musical instrument. Only regular deliberate practice over a long period, with regular expert evaluation and feedback, can bring one to the level where any speech, even one 15 minutes long or longer, can be reproduced in the target language. Again, in all its flavor and personality, tone and emotion, formality or casualness along with facts and figures. And if you're lucky (and good), the speaker will even say (assuming she understands the target language) that your version was better than hers. This does occasionally happen.