Following the Tour de France can be confusing - you have different winners, different jerseys and different contests going on inside the race. This is a quick breakout on how the leaders are determined in the race.
Yellow Jersey: Worn by the rider who has the lowest overall time. Take all the stages and add them up. The lowest time is the winner. The rider wearing yellow at the end of the race is the winner.
Green Jersey: This is worn by the sprinter with the most points. Too keep the race moving, organizers add sprint points throughout the race. These are won by riders who have bursts of speed.
Polka Dot Jersey: Worn by the King of the Mountains (KOM). This rider has the most points gathered from crossing a high point on the race. The points are determined by the climbs difficulty.
White Jersey: This is worn by the youngest rider with the best time.
Some times riders can claim more than one jersey - you could have the highest number of sprint points and have the lowest overall time. In that case the rider usually will wear the higher value jersey, in the case the yellow jersey.