The diatonic scale is most familiar as the major scale or the "natural" minor scale (or aeolian mode).
The diatonic scale is a very important scale. Out of all the possible seven note scales it has the highest number of consonant intervals, and the greatest number of major and minor triads. The diatonic scale has six major or minor triads, while all of the remaining prime scales (the harmonic minor, the harmonic major, the melodic and the double harmonic) have just four major or minor triads.
The diatonic scale is the only seven note scale that has just one tritone (augmented fourth/diminished fifth). All other scales have two, or more, tritones.
The diatonic scale and the melodic scale are the only scales which have just two types of second - the major second and minor second (represented by a whole tone and a semitone, respectively). All the other scales also have a step size of an augmented second as well (represented by a three semitone step), giving them three consecutive-step sizes.
The diatonic scale is also a proper mode. In fact all of the prime scales with the exception of the double harmonic scales are proper modes.
The diatonic scale is therefore an ideal resource for both melodic and harmonic music - it has lots of consonant triads, it has few dissonant intervals, and it is melodically smooth with just two consecutive-step sizes.
The seven modes of the diatonic scale (Lydian, Ionian, Mixolydian, Dorian, Aeolian, Phrygian and Locrian) were (with the exception of the Locrian) widely used in the pretonal Western music of the Middle Ages.