This lesson shows you how to tune your guitar to Drop D tuning, which from low to high (in pitch) is D-A-D-G-B-E.  Drop D is the most common alternate tuning – any tuning other than Standard tuning – and differs from standard only on the sixth string, which is tuned down (or dropped) to D.  Open D is great for playing songs in the keys of D, A, and G.  The tuning has been used by all types of players in all genres for all types of songs, but folkies and hard rock/metal people probably use it the most.  You also see Drop D used in a decent number of guitar arrangements in classical music.