I don't know specifically what the differences are, but usually it involves one or more hardware changes (chipset, change in flash/memory size, processor, etc.). In most cases, these are changes that shouldn't be necessary for the end user to know. The one reason you might need to know is if you intended to update the router w/ third-party firmware. In that case, differences in hardware could affect which firmware worked, if any at all. If you visit the dd-wrt database, it will often provide some details regarding those differences.
http://www.dd-wrt.com/site/support/router-database