WikiHow has a great short article on decoding 12-digit bar-codes:
1 Note that barcodes are made up of both black and white lines. The white spaces in between the black lines are part of the code.
2 Understand that there are three different thicknesses to the lines. Henceforth, the skinniest line will be referred to as "1," the medium-sized line as "2," and the thickest line as "3."
3 Each UPC barcode begins and ends with 111 (thin black, thin white, thin black). In the very middle of the barcode, you will notice two thin black lines sticking down between the numbers. The thin white between them, as well as the thin whites to either side, make up a 11111. Each UPC barcode has 11111 in the middle.
(via Make Blog)
