Barcode scanners are devices which use some sort of light-emitting 
device and a photo conductor to read barcodes. They are usually designed
 to attach to a computer, via either PS2 or some other interface, though
 they may also be used on their own and store the data for future 
retrieval by a computer.
Barcodes are simple visual representations of data which can be 
transferred to a computer via a barcode scanner. Traditional barcodes 
consist of parallel lines, but a number of variations now exist, 
including scatterings of dots and embedded codes hidden within images. 
Barcodes were first conceived in the late 1940s and became widely 
implemented starting in the early 1970s. The first product sold with a 
barcode was a pack of Wrigley's Juicy Fruit gum, at Marsh's supermarket 
in Ohio, one of the first stores to implement barcode scanners. The gum 
now belongs to the Smithsonian and symbolizes a key moment in US 
history.
Barcode scanners typically utilize a laser scanner, though LED and 
imaging varieties also exist. Light is shot out of the barcode scanner 
onto the barcode, then reflected back, picked up by a photodiode in the 
scanner and decoded. The decoded information is then transmitted to the 
computer that the scanner is hooked up to.
Barcode scanners may be completely handheld, in which case they usually 
look almost like a science-fiction era gun, with a laser emitting from 
the barrel, and a trigger to turn the light on or off. These are found 
in some stores, and consumer-level barcode scanners are most often of 
this variety. Barcode scanners may also be built into a surface such as a
 register counter, so that barcodes can be read by swiping them across 
the laser. These types of barcode scanners are most typically found in 
supermarkets because of their ease of use and speed.
Smaller barcode scanners, often shaped like small pens, are also in use.
 This variety is often found in video stores or book stores. Their 
smaller, usually lower-quality light source does not allow for the same 
consistency of reading as larger scanners, but is often adequate for 
simple uses.
A number of barcode standards are in use today, including Code 128, Code
 39, and the Universal Product Code (UPC). Most barcode scanners on the 
market are capable of reading and decoding all of the major barcode 
standards.