In the equal temperament tuning system the octave is divided exactly in twelve notes. The distance between each note is known as a half step. On the keyboard any key is at the distance of a half step from the next and previous key:
A whole step contains two half steps. All the white keys on the keyboard separated by a black key are at a distance of a whole step. The keys that are not separated by a black key are at a distance of a half step:
Notes corresponding to the white keys on the keyboard are called C, D, E, F, G, A and B. These notes are considered natural notes. They can be raised a half step with a sharp or lowered with a flat. A black key, for example the one between C and D, can be considered a C sharp or a D flat:
See Intervals