N = neutral D = drive is the normal mode D = In some cases this serves to limit the maximum speed of the car and thus avoid the consumption of gasoline exes, for example when you are in town and need not go so fast 3,2,1 = yes is like the 1st, 2nd and 3rd in the standard, but the difference is that in automatically when you selected either option "lock" the speed thus avoiding the car passes the next, So if you have selected the car will not pass 1 of the 1st speed and so with the others. This serves mainly on downhill to the car did not go so fast and can stop more easily, in other words is like "engine braking" 
transmission or "automatic" is a gearbox car or other vehicle can handle itself to change the gear ratios automatically as the vehicle moves, thus freeing the driver from the task of changing gear manually. Similar but larger devices are also used in diesel locomotives and machinery for public works, and generally when to transmit a very high torque. Traditionally, the gear ratios are not obtained with parallel gears, as in manual gearboxes, planetary gears but (see picture). By means of suitable hydraulic control devices selectively immobilized one or more components of these epicyclic trains, also called planetary gears
In vehicles, the gearbox and gearbox (also called simply case) is the element responsible for getting the wheels torque enough to put off the vehicle from a standing start, and once up enough to get a couple them to overcome the resistance to the advance, mainly aerodynamic resistance, rolling and slope.The gearbox is because the mission of reducing the speed of the motor as the torque required at any instant. In addition to reverse the direction of rotation of the wheels when the gear needs so require. Is coupled to the flywheel of the engine, which receives motion through the clutch, manual transmissions, or through the torque converter in automatic transmissions. Attached to it going the rest of the transmission system
The transmission is the set of mechanisms to transmit the torque to the wheels, increasing up the courage to start and displacement at any speed, through a series of gear ratios. It also allows the vehicle from a standing start, keep the engine when the vehicle is stationary, and allow such movement back to park. The braking system allows the driver to maintain control at any moment the speed of the vehicle during normal driving and in cases of danger, so that its strength is always higher than the power of traction motor
