Diatonic chords are chords that naturally occur within a key.  There are seven diatonic chords in every key, each chord built from a different note in the scale.  For example, in the key of G major, the G major chord is considered diatonic because it can be formed by stacking thirds starting from “G”: G B D. This is called the I (one) chord in the key of G.  We can do the same thing with “A”, the second note of the G major scale.  A diatonic triad built on “A” is A C E; a minor triad .
Check out the list of major scales to help visualize this!

Diatonic Chords in a Major Key:

It’s common grounds to refer to chords within a key by it’s scale degree in roman numerals.  Generally, major chords will be upper case roman numerals, and minor chords lower case.  In all major keys there will be one diminished chord which occur diatonically.  For this we usually make the roman numeral lower case with “dim” tacked onto the end.  Here are all the diatonic chords within a major key:

I ii iii IV V vi viidim

So, in the key of G major they are:

G Am Bm C D Em F#dim

»Constructing Diatonic Chords from a Minor Scale