Within the last few years, standard UPC/EAN barcodes, such as the ones that this site can produce, have been used to encode ISBNs, that is, the identification numbers that are mostly associated with books and magazines, but also can be used for other media, such as CDs and DVDs.
If you want to encode an ISBN, it's pretty simple:
Start with the 10 digit ISBN, and drop the last digit. (Actually, these days, the last digit is often printed as "X".) This leaves you with nine digits. Add "978" to the beginning of the number. Now you have a 12-digit number. The final barcode will have 13 digits, the last digit being a "checksum" that the Barcode Server will calculate for you if you put a question mark ("?") there. So, enter the 12 digits--just the digits, no hyphens or anything else--followed by the question mark, and you should get the proper barcode.
For example, I am looking at Nolo Press's Fight Your Ticket...and Win!, which has ISBN "0-87337-294-8". The nine useful digits are "087337294". Adding "978" to the beginning, it becomes "978087337294". Adding a question mark to the end, I get "978087337294?", which I type into the Barcode Server input field, and out pops the barcode for that book.
Note that often books come with a five digit supplemental barcode off to the right of the main one (which often represents the retail price of the book). You can encode that with the Barcode Server too, using the advanced options page. Refer to that page for more details.