In the equal temperament tuning system octaves are equally divided into twelve notes. A scale is a series of notes selected among these twelve notes.
Composers write melodies and build chords to harmonize melodies using the scales notes.
We can create our own scales - in the XX century many composers did - but most of the music written since the XVII century uses two types of scales: the major scale and the minor scale.
We can build a major scale using the white keys on the piano keyboard starting with C:
We can build a minor scale using the white keys but starting in A:
But what really defines the type of scale is not its first note but the intervals used to create it....