During GF1 I learned something called 'simple chords' during the very early lessons. Such as fretting the 1st fret second string and playing only three strings to get a C chord. I think there was a G 'simple' chord too. Later I learned the full open chords, A thru G and now I'm wondering what happened to the simple ones?

Are there other 'simple' chords? (B, D, F, etc)
Are these 'simple' chords used or are they just a learning tool?
Do they fit into the 1-3-5 format or do they have their own 'rules'?
Am I getting ahead of myself? :)

Thanks,
GG