Most keyboards contain all the letters of the alphabet (plus some special characters), but not all do, for example calculator keyboards, or the keyboards at a cash machine. Keyboards are the most common form of input device because they are universally available and generally well understood.
The common keyboard is known as the QWERTY keyboard because those are the first six letters on the top line. However, this design is not very efficient, because the arrangement of characters comes from the original typewriter, whose keyboard was arranged to be difficult to use in order to slow typists down to stop the mechanisms jamming. More efficient designs have been produces, such as Dvorak, but they have not taken off because typists would have to re-learn how to type.
A mouse is designed to be used with a pointer on screen. A mouse is really two input devices in one. One is the movement around the screen created by physically moving the mouse. The other is the ability to select using a switch (the mouse buttons). There are several variations of the mouse, for example trackpads on laptops, or the "nipple" on thinkpad laptops.
A barcode reader shines a laser at a barcode (black and white lines), and detects the reflection. It can therefore read the value (normally a number) of that barcode. However, barcodes can easily be misread by the system, so there is normally a checksum at the end. Barcodes are particularly useful, as it allows large amounts of data (such as 13 digit numbers as used in shops) to be read quickly. However, barcodes normally also have the code written below in human-readable form, in case the code won't scan.
Magnetic Ink Character Reader (MICR)
A magnetic ink character reader reads characters which have been printed using special magnetic ink. The value of the characters is readable by both humans and machines. The only common use for MICR is the account information printed on the bottom of cheques.
Optical Mark Reader (OMR)
An optical mark reader can recognize marks on sheets of paper. The position of the mark(s) tells the machine how to interpret the data. Printing on sensitive areas of the sheet is done using a special ink which the scanner cannot "see" - this is why OMR documents tend to be printed in light blue or pink. A standard use for OMR is in multiple choice exam answer sheets, or on a lottery ticket.
A scanner converts a document into an image. The larger the number of pixels in the image, the better the definition of the picture. A typical use for a scanner would be to save family photos on a computer, by "scanning" them in. A scanner can also work with Optical Character Recognition (OCR), to help digitise pages of text, by comparing the individual characters with a database. OCR works best on typed text, but it also works with handwriting.
Screens are categorised according to the number of pixels on the screen. A screen with more pixels will generally have a better picture. The more pixels that there are on the screen, the higher the resolution is said to be.
A special type of screen, called a touch screen, is both an input device and an output device. Not only does it act like a normal screen, but it also acts as an input, which allows it to be used where a keyboard and mouse might not be suitable - for example where the equipment may be vandalised, or with young children.
A printer makes a permanent hard copy of what the user sees on their screen. There are three main types of printers that you should know about:
Thermal printers are often used in shops. They are fairly slow, can only print in one colour, and require special paper, however their main advantage is that they are very cheap to run, and require little maintenance - for example the ink does not need replacing as frequently.
Inkjet printers are cheap printers which produce output by spraying ink onto paper. They produce relatively high quality at a low cost. They can also be used to print photographs, unlike laser printers. They are ideal for home use.
Laser printers produce very high quality prints quite quickly. They are fairly expensive, although recently their price has come down. They have a very low cost per page, which makes them good for office use, where it is important to create a good impression.
Communication devices, such as ethernet and wireless cards (NICs) are a type of peripheral. They are covered in more detail in 1.4.2 Connectivity, and 1.5.2 Network Hardware & Software.