The B2 Super Battle Droid was developed after the Battle of Naboo, when people realized that the regular battle droids were pathetic. Besides being bigger, tougher, and having laser cannons built directly into their arms, the Super Battle Droids were not dependent upon a command ship for orders. This meant that if the command ship accidentally got blown up by a kid flying a Naboo starfighter, the Super Battle Droids would continue to fight, rather than just freezing in place.
Lego Super Battle Droid:
Episode II Super Battle Droid
See Wookieepedia for more information on the Super Battle Droid.
See Bricklink for parts information for today's build.