This is from the Theory section on Chords.

Let's play the most common E minor chord. This is what we call the 'open' E minor chord because it is played at the lowest position on the neck, allowing us to use as many open strings as possible. E minor is one of the easiest chords on the guitar.

All basic major and minor chords consist of 3 notes (the 1st, 3rd and 5th notes), taken from their parent scale. So for the E minor chord we get these 3 notes from the E natural minor scale:

E Minor Scale: E F# G A B C D E

E Minor Chord: E - G - B

Looking at the chord in this ActionTab we can see the notes are played in the following order (from Low E string to high E string):

Low E : B : E : G : B : E

Placing our fingers on these notes and strumming across the strings means we play E, G and B together to get the chord. Of course, some notes we are playing more than once and this adds extra depth to the chord.

This chord may be played using other fingers. For example, if moving from E major to E minor, you would use your 2nd finger on the B (A string), 3rd finger on the E (D string) and then use your 1st finger on the G# (1st fret G string). To quickly change to an E minor chord you would then simply remove your 1st finger from the G string to play open G.

To work out the minor chord triads is always easy as long as you know the major chord. Simply 'flatten' the 3rd note (play 1 fret / semitone lower):

E major triad: E - G# - B
E minor triad: E - G - B

Lowering the 3rd by 1 semitone means you play G instead of G#. The G note is a minor third, thus giving the E minor chord. The 1st and 5th notes remain the same, however there is a significant difference between the sound of major chords and minor chords. This is all to do with the 3rd note used (go back and read this and this if lost).