What is the button w s near the shifter for in a 2001 c240?

The W/S button on the by the shift selector indicates what mode the transmission is in. "W" means winter and starts the car out in second gear to increase traction. "S" means sport and increases the amount of time (shown to you in RPMs) the car stays in a particular gear. "S" is designed to make the gear ratiOS taller, therfore adding to performance.