the FRS and GMRS units are nearly identical, except the GMRS has some additional frequencies which work for further distances (the FCC allows you to use higher wattage signals I believe).
The FRS and GMRS units use the regular FRS allocated frequencies so they can communicate with those popluar handheld walkie-talkies.
The GMRS unit only has 15 channels, not the full 22 or so. I believe FRS was 14 channels and GMRS adds an additional 8.
Technically (legally) you are required to register with the FCC to use the GMRS frequencies, but few people seem to bother with this. Use of the FRS frequencies requires no registration.
They don't work great at high speeds, but that will certainly vary from helmet to helmet depending on how much wind noise you get.
They both have audio in so you can listen to music.
They also have a connector that works with cellphones using the standard 3pin connector... Most nokias use 4 pins and it didnt work for me.
You can buy a second headset to install in your passengers helmet and then use a standard keyboard extension cable to connect it to the main box for rider to rider communication.
These units are essentially just fancy FRS/GMRS walkie-talkies. The one difference in the chatterbox units is that the channels are not the same.. I believe channels 1-7 on the chatterboxes are the same as channels 1-7 on most (if not all) of the handhelds, but after that channel 8 on the chatterbox is like 16 or something.
I originally bought a different unit, the name of it escapes me at the moment, and it wasnt all that great. You had to buy the walkie talkie seperately and it had cables coming out from all over the place like an octopus.
If you have specific questions, please ask.