A Diatonic is the very basic scale, really just the sequencing based on tones & semitones. For example. Key C: C-D-E-F-G-A-B-C...the sequence will be W-W-h-W-W-W-h ...W=Whole step or tone and h+half Step or semi-tone.
Example: Do-Re-Mi-Fa-So-La-Ti-Do