Hey Friend,
E minor has a substantial role in the key of G major. As the six chord (VIm), it will get used a bunch in songs. In Rock , Blues and Country, the six chord is used very often to create tension in the chord progression.
It's a very common occurrence to see a song using the I, IV and V chords in various arrangements, and there are tons of gold records for songs that used only the I, IV and V chords. But since these chords are pretty resolved on their own, songs written with the VIm chord added to the mix help give a song more emotional movement. The progression changes from being primarily a single emotion to moving between two or more emotions.
We looked at E minor during the study of the C major key, and if you recall, the notes are 1 3m 5, or E, G and B. These fall within the key of G as well:
G A B C D E F#
The chord is constructed using the notes from the key of E major, and then lowering the 3 note a half step to make the chord minor.
E F# G# A B C# D#
E G B
See how G# became G when you move down one fret on the guitar neck? Check out the exercise files and let me know what questions you have.
See you tomorrow!
Peace~
Dave | 
