When you place the shifter in 'M' the car is not automatically in Manual mode, it's in Sport mode. Once you hit one of your paddle shifters, it switches to Manual mode. If you get tired of paddle shifting, you can push the shifter back up into 'D' and back into 'M' to go back to Sport mode.
The difference between Sport mode and Drive is that Sport mode has a different set of shift points and is more aggressive in that respect. It will hold a gear a little bit longer on acceleration and downshift quicker during decceleration.
Once you hit the paddle shifter, though, the shifting is totally up to you. Though, if you're in 3rd, 4th, 5th, or 6th and come to a stop, the car will automatically drop you down into 2nd.
Hope that helps
