A computer keyboard consists of alphanumeric or character keys for typing, modifier keys for altering the functions of other keys, navigation keys for moving the text cursor on the screen, function keys and system command keys—such as Esc and Break—for special actions, and often a numeric keypad to facilitate calculations.

There is some variation between different keyboard models in the physical layout—i.e., how many keys there are and how they are positioned on the keyboard. However, differences between national layouts are mostly due to different selections and placements of symbols on the character keys.
The core section of a keyboard consists of character keys, which can be used to type letters and other characters. Typically, there are three rows of keys for typing letters and punctuation, an upper row for typing digits and special symbols, and the Space bar on the bottom row. The positioning of the character keys is similar to the keyboard of a typewriter.
Besides the character keys, a keyboard incorporates special keys that do nothing by themselves but modify the functions of other keys. For example, the ⇧ Shift key can be used to alter the output of character keys, whereas the Ctrl (control), Alt (alternate) and AltGr (alternative graphic) keys trigger special operations when used in concert with other keys. (Apple keyboards have differently labelled but equivalent keys, see below).

Typically, a modifier key is held down while another key is struck. To facilitate this, modifier keys usually come in pairs, one functionally identical key for each hand, so holding a modifier key with one hand leaves the other hand free to strike another key.

An alphanumeric key labelled with only a single letter (usually the capital form) can generally be struck to type either a lower case or capital letter, the latter requiring the simultaneous holding of the ⇧ Shift key. The ⇧ Shift key is also used to type the upper of two symbols engraved on a given key, the lower being typed without using the modifier key.

The Latin alphabet keyboard has a dedicated key for each of the letters A–Z, keys for punctuation and other symbols, usually a row of function keys, often a numeric keypad and some system control keys. In most languages except English, additional letters (some with diacritics) are required and some are present as standard on each national keyboard, as appropriate for its national language. These keyboards have another modified key, labelled AltGr (alternative graphic), to the right of the space bar. (US keyboards just have a second Alt key in this position). It can be used to type an extra symbol in addition to the two o…

Read more on Wikipedia

Keyboard layouts have evolved over time, usually alongside major technology changes. Particularly influential have been: the Sholes and Glidden typewriter (1874, also known as Remington No. 1), the first commercially successful typewriter, which introduced QWERTY; its successor, the Remington No. 2 (1878), which introduced the shift key; the IBM Selectric (1961), a very influential electric typewriter, which was imitated by computer keyboards; and the IBM PC (1981), namely the Model M (1985), which is the basis for many modern keyboard layouts.

Within a community, keyboard layout is generally quite stable, due to the high training cost of touch-typing, and the resulting network effect of having a standard layout and high switching cost of retraining, and the ubiquity of the QWERTY layout is a case study in switching costs. Nevertheless, significant market forces can result in changes (as in Turkish adoption of QWERTY), and non-core keys are more prone to change, as they are less frequently used and less subject to the lock-in of touch-typing. The main, alphanumeric portion is typically stable, while symbol keys and shifted key values change somewhat, modifier keys more so, and function keys most of all: QWERTY dates to the No. 1 (1874)—though 1 and 0 were added later—shifted keys date in some cases to the No. 2 (1878), in other cases to the Selectric (1961), and modifier key placement largely dates to the Model M (1985); function key placement typically dates to the Model M, but varies significantly, particularly on laptops.

The earliest mechanical keyboards were used in musical instruments to play particular notes. With the advent of the printing telegraph, a keyboard was needed to select characters. Some of the earliest printing telegraph machines either used a piano keyboard outright or a layout similar to a piano keyboard. The Hughes-Phelps printing telegraph piano keyboard laid keys A-N in left-to-right order on the black piano keys, and keys O-Z in right-to-left order on the white piano keys below.

In countries using the Latin script, the center, alphanumeric portion of the modern keyboard is most often based on the QWERTY design by Christopher Sholes. Sholes' layout was long thought to have been laid out in such a way that common two-letter combinations were placed on opposite sides of the keyboard so that his mechanical keyboard would not jam. However, evidence for this claim has often been contested. In 2012, an argument was advanced by two Japanese historians of technology showing that the key order on the earliest Sholes prototypes in fact followed the left-right and right-left arrangement of the contemporary Hughes-Phelps printing telegraph, described above. Later iterations diverged progressively for various technical reasons, and strong vestig…

Read more on Wikipedia

Although there are a large number of keyboard layouts used for languages written with Latin-script alphabets, most of these layouts are quite similar. They can be divided into three main families according to where the A, M, Q, W, Y and Z keys are placed on the keyboard. These layouts are usually named after the first six letters on the first row: AZERTY, QWERTY, QWERTZ, QZERTY and national variants thereof.

While the central area of the keyboard, the alphabetic section, remains fairly constant, and the numbers from 1–9 are almost invariably on the row above, keyboards may differ in:
• the placement of punctuation, typographic and other special characters, and which of these characters are included,
• whether numbers are accessible directly or in a shift-state,
• the presence and placement of letters with diacritics (in some layouts, diacritics are applied using dead keys but these are rarely engraved).
• the presence and placement of a row of function keys above the number row
• the presence and placement of one or two Alt keys, an AltGr key or Option key, a backspace or delete key, a control key or command key, a compose key, an Esc key, and OS-specific keys like the Windows key.
The physical keyboard is of the basic ISO, ANSI, or JIS type; pressing a key sends a scan code to the operating-system or other software, which in turn determines the character to be generated: this arrangement is known as the keyboard mapping. It is customary for keyboards to be engraved appropriately to the local default mapping. For example, when the ⇧ Shift and numeric 2 keys are pressed simultaneously on a US keyboard; "@" is generated, and the key is engraved appropriately. On a UK keyboard this key combination generates the double-quote character, and UK keyboards are so engraved.

In the keyboard charts listed below, the primary letters or characters available with each alphanumeric key are often shown in black in the left half of the key, whereas characters accessed using the AltGr key appear in blue in the right half of the corresponding key. Symbols representing dead keys usually appear in red.
Turkmen keyboards use a layout known as ÄWERTY (/æˈvɜːrtiː/ av-UR-tee), where Ä appears in place of Q above A, Ü appears in place of X below S, Ç appears in place of C, and Ý appears in place of V, with C, Q, V, and X not being accessible at all. It is supported by Microsoft Windows (Vista and later only).
The AZERTY layout is used in France, Belgium, and some African countries. It differs from the QWERTY layout thus:

Read more on Wikipedia