The core has an extra layer. It is also slightly larger... by 4mm I think. And it runs on a lower voltage. All of these mean easier cooling and better-suited for OEM stock speed machines, and also better OC'ing overhead.
To also add to what has already been posted.
According to the technical specs on the AMD site TBred A's come in flavours from XP1700 - XP2200, Tbred B's start at XP1800 then skip XP1900 and range from XP2000+ upwards.
The Thermal Power and Processor Current is higher on the B's (example for the XP1800+ the typical thermal power is 55.7W on the B and 46.3W on the A and the typical Processor Current is 34.8A on the B and 30.9A on the A - note the XP1800+ shows the greatest difference for all the comparable speeds)
However for the Vcore it can be higher, lower or the same! Example the XP1800 for the A the Vcore is 1.5V but for the B it is 1.6V, for the XP2000 it can be 1.6V or 1.65V for the A but 1.6V for the B, for the XP2200 it is 1.65V for the A but 1.6V for the B! For XP2400+ and higher it is 1.65V.