Western music divides the octave into 12 equal semitones. Every scale, chord and interval is built from selections of these 12 notes.
Between one A and the next lie eleven other notes: the white and black keys of one piano octave. The step between neighbors is the semitone, the smallest standard distance in Western music.
Two semitones make a whole step. Familiar melodies are mostly built from whole and half steps, and the pattern of those steps is what gives each scale its identity.
Common questions
Why does Western music use 12 notes?
Twelve equal divisions of the octave approximate the most consonant frequency ratios, especially the 3:2 perfect fifth, closely enough that all 12 keys work equally well.
What is a semitone?
The smallest standard distance between two notes in Western music: one twelfth of an octave, the step from one piano key to its immediate neighbor.