Captions, composed of text, are used by people who are deaf or hard of hearing to access content delivered by spoken words and sounds. Real-time captions, or Computer Assisted Real-time Translation (CART), are created as an event takes place.
A captioner (often trained as a court reporter or stenographer) uses a stenotype machine with a phonetic keyboard and special software. A computer translates the phonetic symbols into captions almost instantaneously and displays them on a laptop or on a large display screen. A slight delay may occur because of the captioner's need to hear and enter the words and the computer's processing time. Real-time captioning can be used for programs that do not have written scripts or captions such as: lectures, classes, congressional or council meetings, news programs, and non-broadcast meetings, such as those of professional associations.
Remote real-time captions are produced at a remote location and then transmitted to the site where the program is taking place. For example, in a lecture hall an instructor can talk into a microphone that is connected via telephone lines to a captioner in a different city. From that location, the captioner, using similar equipment as described above, transmits the captioned text via the internet, using special software, to a laptop in the lecture hall- or to a laptop in a student's home, if they are unable to attend the lecture in person.
Although most real-time captioning has been estimated to be well over ninety percent accurate, the audience will see occasional errors. The captioner may misunderstand a word, hear an unfamiliar word, or there may be an error in the software dictionary.
Captions can also benefit individuals who understand the written better than the spoken word of the language in which a presentation is delivered as well as people who are viewing the program in a noisy (e.g., airport or sports bar) or noiseless (e.g., a work cubicle) environment. Captions that are not "real-time" include those provided on television programming and those made available on prerecorded video that can be rented or purchased.
For more information on captioning consult the DO-IT Knowledge Base articles How can people who are deaf access video and multimedia products? , Is there a way to add captions to a video that is already published? , and Where can I find vendors who provide captioning services? 
-  How can people who are deaf access video and multimedia products?
-  Is there a way to add captions to a video that is already published?
-  Where can I find vendors who provide captioning services?