They can hold a lot more data than the single numeric barcode we use to identify library books. They're generally read from the center out, rather than the linear right to left of the barcodes used by libraries. They grow in size depending on the amount of data encoded and are based on a square grid.I chose the Aztec code for my example, but there are many others. And I suspect we will be seeing them more and more as the code is well suited for display on cell phones and other mobile devices.
I found a free barcode generator online and just to experiment put in my name, title, organization, city, state and ZIP code. Here's what it looks like along with a plug for the generator's company.
TEC-IT Barcode Software