My answers:

a. What does RealBand do ?
A. It does much of what BiaB does in a more linear way. It also records more tracks (48 available), allows more generated tracks, and some other features unique to it. For people who are used to working in a linear, multitrack environment it is more intuitive. For some users that are more 'Chorus repeat' oriented, Biab may be more comfortable. I'm not comparing it to Sonar or Pro Tools, but Realband is a lot more like one of those programs than BiaB is. They are different. How you like to work and what you want to do will dictate which is better for you. Tell us more about what you want to do with them, then we could better explain how they would different for you.

b. In order to use RealBand, do I need BIAB ?
A. You need BiaB, since it is the only way to own Realband. Realband is very unique in that it is not sold on its own. It has to have BiaB to 'rely on' for some stuff, but expands BiaB in a different environment.
c. If I decide to use RealBand, is there an additional cost, and if so, what is the amount ?
A. No extra charge; you own Realband if you own a recent copy of BiaB; it is included. Which is why in the question above the answer is you need BiaB .. you don't have to use it though! They come as a package because Realband relies on BiaB code for the BiaB features. *Note - you can buy Realband without the BiaB features; it is sold on this site as 'Powertracks'. It is simply a 48 track DAW.
d. Is there a glossary for all the unknown terms forum members are using ? I see terms that I have trouble finding in the PGMusic "pdf" manuals.
A. You may be better off to just ask here on the forums. Terms used here are sometimes best explained here.

I do not work here, but the benefits are still awesome
Make your sound your own!