The basics
Major & Minor Chords
A chord is when three or more notes are played at the same time. The two simplest kinds of chord are 'major' and 'minor'. You can take any note then add the notes that are a major 3rd and a perfect 5th above.
A major chord then is made up of a root, major 3rd and perfect 5th.
The example chords are written in triad form. The triad clearly demonstrates the theory but is not necessarily how they would be played on the guitar. Playing these chords on the guitar is covered later.
In practice, A major would simply be written as A, and C major would be written as C.
A minor chord is made up of the following: root, minor 3rd and perfect 5th.
The standard way to write minor chords is as Am, Cm etc.