Play a chord of three notes on a piano. E.g. C major. That means you play C-E-G. Now count the number of piano keys from C to E. You'll notice its 5 keys. Now count from E to G and you'll notice its four keys.
Written down these notes look to be evenly spaced apart but they're not really. The gap of four notes is a 'minor' and the gap of five notes is a 'major'. If the major is closest to the C (the tonic or note that gives the chord its name) then its a Major chord. If that first gap is only four keys then its a Minor chord.
If you have two five key gaps in the chord its called an 'Augmented' chord and if you have two lots of four keys its a 'Diminished' chord.