B&W line up

In general, the older B&W 800 series has warm and full sound but less high resolution transparency.  Each newer generation uses better crossover components and speaker cone materials to increase the clarify and resolution but in a compromise becomes loses the warmth and fullness.  The biggest difference in the latest D3/D4 generation are the shocking price tag increase and that the even the smallest floor stander 804 D4 can generate sub bass frequency going all the way to 24hz (@-3db) and 20hz (@-6db).  This is very difficult to reproduce by other speaker manufacturers without using huge 10+" bass woofers.

I can help here.  I'll keep it simple and stick to the 800 series.  Essentially the smaller the number, the bigger the speaker.  805 is the smallest bookshelf speakers of the 800 series (been this way since the early 90s) then 804 is the smallest floor stander and 800/801 are the rarest and largest floor standers of the 800 series.

Series wise, starting with the 800 Matrix series in the 90s, followed by the Nautilus series, then the S series, then the D (aka. Diamond) series, then D2 (aka. Diamond 2), D3 and the latest series is called the D4.