In computing, a keyboard is a typewriter-style device, which uses an arrangement of buttons or keys, to act as mechanical levers or electronic switches. Following the decline of punch cards and paper tape, interaction via teleprinter-style keyboards became the main input device for computers.
Source: Wikipedia [http://en.wikipedia.org/wiki/Keyboard_(computing)]
Computer keyboard include engraved or printed characters which allow to be identified during the writing process. In addition, characters' size meet standardization criteria and is suitable for most of the population. Nonetheless, visually disabled people might find that characters' size and contrast are not enough clear to be identified by the user.
Besides, there are some models of keyboards which have magnified characters filling all the space in the key, in order to be as big as possible to improve reading.