Barcodes

Also:

Javascript Barcodes from RenderBar

Technical details

David B. VanHorn has posted an excellent tutorial on how to write your own barcode decoding software: http://dvanhorn.org/Barcode/. (He uses Atmel AVR, but the tutorial is general enough to apply to just about any processor). Sadly, Davids website has been taken down. I've copied what I could find in caches to this page.

Glossary
Basics (how to read barcodes from a light input)
EAN 13

Glossary

Distortion:: An element is distorted when it is not printed accurately. In other words, elements are always distorted. The theoretical limit for distortion in UPC/EAN is 1/2 module.
Element:: A black or white bar
First Read Rate:: The percentage of time that a correct code will be output on the first scan, provided that there are no mechanical problems with the scan (not scanning the entire code for example) and that the barcode is printed within specifications.
Forward:: Scanning the barcode from left to right.
InkSpread:: The tendency, in most printing methods, for black bars to be wider than they are intended to be. Usually a positive number, representing the relative width of black bars to white bars. If we scan equal bars and see 20 counts for white and 22 counts for black, then the spread would appear to be 2 counts or 10% The light pen itself may also be biased slightly, usually favoring black.
Light Pen:: A device that indicates only the presence of "black" or "white" as a binary level on a single pin/wire. A light pen does not process the data other than to decide on what level of reflectivity qualifies as "black" or "white".
Misread:: See Substitution
Module:: A standard unit of width. An element's width is defined as N*Module.
No-Read:: A failure to decode, resulting in no output. These are annoying to the operator, but relatively harmless. Ideally, if the scan is done properly, and the barcode printed within specifications, the No-Read rate should be near zero.
Scan:: The data collected in an entire pass across a barcode. May include various noise on either end.
Substitution:: Outputting a digit improperly, for example, outputting a six when a nine was scanned. Substitution errors point to a weakness in the cleanup algorithms, or in the symbology itself. Interleaved 2 of 5 is particularly weak.
Symbology:: A particular system of encoding information in a barcode. UPC and Code 39 are two popular symbologies. Codabar, Code 93, Code 128 are some others.