The yellow jersey - The leader's jersey, based on the fastest cumulative time among all the riders.
The green jersey - Commonly called the sprinter's jersey. It's actually the "points" jersey, and is awarded to the rider who accumulates the most points by being the first one past designated spots on the course, and at the finish.
The polka-dots - Also known as the "climber's jersey," is won by accumulating bonus points on designated spots on climbs, as well as mountain finishes. The jersey is contested only when at least some climbing is involved.
The white jersey - The "best young rider jersey", based on fastest cumulative time -- but for riders under 25.