The chuck that attaches the pump to the valve are different. The schrader valve includes a spring that holds the valve shut. The chuck for the schrader valve has a pin in the middle that holds the valve open while the chuck is attached. The presta valve is held closed by the pressure in the tire, but the valve has a little nut that holds it shut when not being pumped. You have to loosen this nut before you attach the pump chuck. Some pumps have chucks that have to be taken apart to convert from one style to the other, others have double heads, and some chucks recognize the valve type as you put the chuck on and reconfigure themselves accordingly.