D-Star

It's APCO for hams! It is really cool. You can surf the web, check email, TXT message, send your postition, and much more, ALL DIGITALLY! And listening to it on Analog is cool.

D-PRS

It is like APRS, but Digital, Duh... Right? I would say it is much easier, than APRS in the way that you don't have to buy a TNC. All you need is a GPS receiver, and make/buy a cable. Most D-Star radios have a diagram in the manual. Program in a few GPS sentences, aquire GPS signal, and your ready.

D-Chat

There are a couple of them out there, but I believe this is the most common one. TXT messaging from computer to computer. You can also TXT just from the radio too.