Digitizer?

I was checking out the Wacom tablets to replace my current Aiptek one (nice for starters but not a lot of flexibility) and I noticed that there seemed to be some diferences in the naming conventions, even amoung some of the same versions of tablet.

In this case, some of them had the term Digitizer in the name and I didn't see any ready explination of what that was about. Does anyone here know the difference? I thought that they are all digitizers after a fashion.