The keyboard is, for the majority of computer users, the prime method of data entry and interaction with the machine.
Its design becomes very important when you realize how many hours you spend pounding at it.

Keyboards could be classified (if you where an insane UI person and delighted in boring tasks) along many lines of which the most important are:

  1. Key layout: The spatial disposition of the keys is very important in blind typing. The most common (and I am talking about keyboards for Latin-based alphabets, excluding Russian, Arabic and other alphabetic scripts) is without doubt the QWERTY keyboard, followed by the Dvorak keyboard. There are many national variants, designed to accomodate the special characters for áccèñtéd lëttèrs and local currencies. In fact, keyboard layout is a huge mess, whith every producer doing its little bit to confuse the matter and making keyboard mapping (the province of programs like xmodmap) a daunting task.
    Also, keyboard layouts differ in the number of function keys, the size of keys like Enter, Tab, Shift, Alt, Control, Delete and Backspace, and also in the presence and shape of a numeric keypad.
  2. Key technology: there is a good chance that the keyboard you are using right now has a little electrical switch underneath every key. The mechanics of said switch make up for a large part of the "feel" of the keyboard. Switches vary in clickiness, loudness and force required to actuate them. Usually you want a strong initial resistance, a lower but constant resistance in the travel phase, and a firm "bottom" end when travel is completed. Usually the keyboard clicks when the key is released, with the exception of the IBM clicky keyboards that click twice (a great ego booster).
    Another keyboard technology (membrane keyboard) uses little "bubbles" in a plastic film. This type of switch has advantages: it is cheap to produce, easy to make water-proof and it can be printed on. But it has one major defect, that's to say it does not move very much when pressed. The user remains in doubt, because there is little tactile feedback. I have heard this kind of keys compared to "little dead sea animals".
    This makes blind typing difficult, and in fact restricts this kind of key to devices that have to operate in hostile environments (with water and grease and dust ...). It was also seen in some home computers like the Sinclair ZX Spectrum and the ZX 81 (in those cases it was combined with a data entry system guaranteed to drive you nuts).
  3. Keyboard shape: it has been observed that the standard keyboard shape (rectangular, keys in a row) forces your wrists to be in a angled position for hours at a time. This is supposed to be bad.
    An answer to this problem are the various "split" designs in which the keyboard is divided into two parts, one per hand. Pioneer in this field was Apple, with a split keyboard where you could modify the angle and distance between the two parts. The keyboard was not a huge success, and Microsoft proceeded to snap up the idea, make it cheaper to produce, and turn it into a success, with the Microsoft Natural Keyboard: notice that the two parts are rigidly connected.
    This design approach reaches the extreme of splitting the keyboard into two parts that you are supposed to bolt to the armrests of your chair. Very Star Trek.
Improper keyboard design and use are a big source of RSI, including the carpal tunnel syndrome.
An alternative idea is the chording keyboard, usually having a small number (e.g. five) of big keys. Characters are formed by playing a chord. This reduces wrist motion to nil, and allows incredible typing speeds. This kind of keyboard is used on shorthand machines like those used in courts.

see also: AT Keyboard Enhanced AT Keyboard