Going from a C to a C# is a half step, while going from a C to a D is a whole step. If you have a piano at hand, it's easier to explain.
If you look at the C major scale on the piano, it is all white keys. Starting on a C, it's a whole step to a D, whole step to E, half step to F, whole step to G, whole step to A, whole step to B, and a half step to C. Notice that between the E and the F, and the B and the C(the half steps), there is no black key between those notes. Hopefully it makes some more sense now.