D stands for ‘Drive’ and is the mode you need to select (using the gear lever or selector) to go forward. In D mode, the car will choose when to change up and down. To maximise fuel economy it will, for the most part, give preference to higher gears and lower engine speeds.
S stands for ‘Sport’ and is designed for those times when you want to accelerate more quickly or simply give the car a sportier feel when driving. The gear changes happen at higher revs, which generally makes the car respond quicker when you put your foot down.