Half step
A half step (also called a semitone) is the smallest distance between two notes: from a key to the very next key, whether black or white. There are twelve half steps in an octave.
From C to the black key just to its right is a half step. From that black key to D is another. Between E and F, and between B and C, there is no black key, so those white keys are also a half step apart. Count keys, not colours.
Two half steps make a whole step. Scales are built from both: the major scale pattern is whole, whole, half, whole, whole, whole, half. Its half steps fall where the white keys have no black key between them, E–F and B–C.
A sharp raises a note by a half step, a flat lowers it by a half step. In interval names, a half step is a minor second.
What it sounds like
A half step sounds tense, as if the two notes were rubbing against each other. Two notes a half step apart, alternating, make the menacing theme from “Jaws”.
Where it is taught: the studio’s interactive textbook, chapter 5, “Half steps, whole steps and the black keys”.