In general the the four digit cameras like the xxxxD are basic entry level models, the three digit ones xxxD have better noise performance and more features like mirror lock up, the two digit ones are usually a more rugged have many more features like micro focus adjustment for lenses etc, the single digit cameras are the pro models..
The first number is usually and indication of manufacture date so the series might go something like 300D/400D/500D etc with all the models having similar design features but big improvements in performance with each step..
Alan