Robin hood is correct - SMG is an electronically controlled Manual gearbox that can shift faster than any manual. There's no torque converter as on the automatics/steptronic. The newest M5 has the generation 3 SMG that is the fastest yet.
US M5 owners and owners to-be in the states demanded the manual, so BMW catered to the largest market by providing one. Take a look at the M5 boards.
E90 TiAg 330i ZSP, ZPP, ZCW 6sd Manual, Nav & Satellite (for Stern!), Black leather w/ Poplar