usually, people compare the A4 with the 3 series, A6 with the 5 series, and the A8 with the 7series.
There is also a Audi Allroad, TT, A2, and A3 (sportsback).
Q7 is also in plans for production soon.
basically, there was only the RS4 (b5, b7), and the RS6. These are one step up from the S versions.
when people mention b5, c6 or D3, the alphabet means the chassis size/type, and the number means the revision.
For the B5 A4 (the one in my sig), there is an S4 with a 2.7tt engine, comes in sedan or avant(wagon).
The B5 RS4, which has more power, comes only in advant form, and has a widebody plus lots of goodies.
There is no b6 rs4, only a b6 s4. comes with a NA engine.
the B7(latest) S4 is more or less similar to the B6 s4.
B7 RS4 is the talk of the town now, a 4.2 V8 that goes to 8k rpm, tonnes of torque, awesome interior..bla bla bla...
http://video.google.com/videoplay?docid=-5252945100827089880&q=rs4
same story with the S6.
The C5 chassis has got a RS6, which was once the most powerful audi ever produced.
The new C6 s6 which was just announced recently is supposed to have a V10.
there was also the S8.
given you enough names already, google them for yourself.