In most Western music, the smallest distance (interval) between two pitches is a half step. This diagram shows part of a piano keyboard. Two keys with consecutive numbers are a half step apart. For example, key 2 and key 3 are a half step apart.
Two half steps combined make a whole step. Looking at the piano keyboard, you will notice a black key between G and A. The distance from G (8) up to the black key (9) is a half step. From the black key (9) to A (10) is another half step. The combined distance (two half steps) is a whole step (from G to A).
Notice the various half and whole steps on the keyboards in the following diagrams:
It is important to observe that there are no black keys between E and F and between B and C. These are called natural half steps and must be memorized.