what is the difference between them?
what year is B5, B6, and B7?

B5 is 1996-2001.5
1996 only the 12 valve V6 was available
1997 the 1.8T was introduced, the 12v was still there too
1998 the 30 valve V6 was introduced and so was the avant model
1999.5 got a facelift...1 piece lights different trim, etc.
2000 drive by wire is implemented. 2.7T S4 is introduced.
2001 got the K03 sport turbo 170 hp emgine and the red thing next to the "T"

I dont know a REAL lot about the B6 but here goes..
B6 is 2002-2004 -S4 is now a 4.2 V8 for 03 model, B5 S4 ran until 02
2002 6 speed manual was an option, 2.8 30v becomes 3.0
2003 6 speed manual is only manual tranny, and CVT is introduced
2004 ultrasport is avail (might have been in 03 not sure?)

B7 is 2005 (or maybe 05.5?) and up
1.8T is replaced by 2.0T and 3.0 V6 is replaced by 3.2 FSI V6.
RS4 is schedueled to come with a 420 hp N/A 4.2

This is for the united states, everything was introduced in europe about a year earlier.

I may ave made a few mistakes, but overall its pretty close