British and American keyboards

There are two major English language computer keyboard layouts, the United States layout and the United Kingdom layout defined in BS 4822 (48-key version). Both are QWERTY layouts. Users in the United States do not frequently need to make use of the £ (pound) and € (euro) currency symbols, which are common needs in the United Kingdom and Ireland, although the $ (dollar sign) symbol is also provided as standard on UK and Irish keyboards. In other countries which predominantly use English as a common working language, such as Australia, Canada (English speaking parts), Pakistan, India, and New Zealand, the US keyboard is commonly used.
Windows keyboards.
The UK variant of the Enhanced keyboard commonly used with personal computers designed for Microsoft Windows differs from the US layout as follows:
Early versions of Windows handled both the differences between the two keyboards and the differences between American English and British English by having two English language options — a UK setting and a US setting. While adequate for users in the United States, United Kingdom, and Ireland, this solution caused difficulty in other English-speaking countries. In many English-speaking jurisdictions (e.g., Canada, Australia, the Caribbean nations, Hong Kong, Malaysia, India, Pakistan, Bangladesh, Singapore, New Zealand, and South Africa), orthography has traditionally conformed more closely to British English usage, while these countries have chosen to use the United States keyboard layout. People in these countries were as a result required to choose a system setting inconsistent with British English, thereby causing traditional British English to fall out of favour. This is particularly evident with spelling, where words such as "colour" and "centre" are flagged as being spelled incorrectly by word processing software when the operating system is set to the US setting.
However, in more recent editions of Windows, the number of 'settings' options was increased, allowing users to select the correct keyboard and dialect independently. For example, one is given a number of default options for locality that will usually correctly match dialect and keyboard. Further, even if the hardware keyboard layout does not match the region that was pre-selected, it can be changed without changing the regional setting.
International or extended keyboard layouts.
Since the standard US keyboard layout in Microsoft Windows offers no way of inputting any sort of diacritic or accent, this makes it unsuitable for all but a handful of languages unless the US International layout is used. The US International layout changes the (grave), (tilde), (circumflex), (double quote, to make diaeresis), and (apostrophe, to make acute accent) keys into dead keys for producing accented characters: thus for example  (release)  will produce . The US International layout also uses the right alt (AltGr) as a modifier to enter special characters. 
The equivalent mapping for UK/Irish keyboards is called the "UK Extended" layout which, if activated in settings, will allow the user to enter a wide variety of diacritics (such as grave accents) which are not accommodated by the standard UK/Irish layout. In particular, à,è,ì,ò,ù used in Scots Gaelic can be made (using , release and then the vowel), the ŵ and ŷ used in Welsh (using (^), release, then etc.). Likewise, the Spanish and Portuguese letters ñ and õ can be made (using (~), release, then etc). 
For more specialized uses, there is a facility in Windows for users to create a customized layout that may match their needs more precisely.
Apple Macintosh keyboards.
The non-standard default U.S. layout on Apple Macintosh computers allows input of diacritical characters, whereby the entire MacRoman character set is directly available.
Apple supplies a "British" keyboard layout with the following differences:
Other keyboard layouts.
Other operating systems can optionally re-map the keyboard layout or have different modifier keys (for example the Amiga keyboard has "A" modifier keys and BBC Micro or Acorn keyboards often had a "Shift Lock" as well as a "Caps Lock").
Under Unix/Linux the "Windows" key is often called the "Super" key and can be re-mapped by users for specific functionality but in most programs by default does nothing.
Some older Unix/Linux software, such as Emacs, uses the left Alt key as a "Meta" key, which harks back to older MIT or LISP computers.
Chrome OS uses the US and UK Windows layouts, except that the Caps-Lock key is labelled with a magnifying glass (🔍) and by default acts as a web search key (there is an option in Settings to revert it to convention). The function-key row is also differently labelled. The UK-extended layout for Chrome OS is provided by a Chrome add-on, and provides ready access to a substantially greater (than Windows) repertoire of precomposed characters for western, central and eastern European (Latin alphabet) languages. 
Dvorak layouts.
There are also Dvorak Layouts for each region.