Diatonic Scales

A diatonic scale is not a specific scale, but rather a way a scale (or a chord for that matter) is constructed. A diatonic scale is constructed from a mix of whole and half steps in a contrary way to chromatic scales which are constructed only by half steps.

Below, a chromatic scale (C Chromatic).
C Chromatic scale diagram

Below, a diatonic scale (C Major).
C major scale diagram

There are both diatonic scales and chords. The term diatonic is going back to the ancient Greece, where musicians separated octaves into intervals with names that all begun on dia-.

A diatonic scale is built on the intervals made by natural notes (i.e. neither flat nor sharp) and based on seven whole steps of perfect fifths: C - G - D - A - E - B - F. In modern Western music a scale is referred to as diatonic if it is based on five of whole steps together with two half steps.

The Major Scale has this formula: Whole, Whole, Half, Whole, Whole, Whole, Half. The same is true for the formula of the Natural Minor: Whole, Half, Whole, Whole, Half, Whole, Whole.

Many scales are diatonic including major, minor (the Harmonic minor is an exception) and modal scales. Examples of non-diatonic scale are pentatonic, octatonic and whole-tone scales. Also, as a more obscured example, the Acoustic scale.

The thing to remember is that diatonic is a central term in music theory that can describe to scales and chords. Diatonic can relate to scales which consists of seven tones that can be ordered in some specific way, for example, by five whole tones and two half steps. It can also describe chord which are all related to the same major or minor key. For example, the chords C, Dm, Em, F, G, Am and Bdim are diatonic chords based on the C major scale. See Harmonizing major scales into chords for more examples.

Sources:
Music and Twentieth-Century Tonality: Harmonic Progression Based on Modality and the Interval Cycles (Routledge Studies in Music Theory) by Paolo Susann and Elliott Antokoletz

Wikipedia.org, https://en.wikipedia.org/wiki/Diatonic_scale