The octave is divided into twelve equally spaced notes.  If you played all of these notes you would be playing a chromatic scale.  Other music traditions like Chinese divide the octave differently so they have different scales than western music.

The word octave comes from the Latin meaning "eight".  So while we have twelve notes between octaves only seven are used in any particular major or minor scale.  the eight note is the octave above or below the beginning note.  These are "Diatonic" scales and they form the basis for most western music.  It is much easier to visualize using a piano keyboard where all white keys represent the seven letter notes of A,B,C,D,E,F,G, and the black keys represent the sharp or flat of each note.

Whether it is a pop song, a classic symphony or an old folk tune, most music that feels comfortable to western listeners is based on either a major or minor scale.  The other notes on the chromatic scale are used to sparingly to add interest or to change the key in the middle of the music.