Summarize the following:
Some guitarists tune their E string down to D so that they can play power chords. This is seen by many as cheating, but deliberately 'mistuning' strings (scordatura) for any reason is a centuries old method used for any stringed instrument and has been used by Van Halen, Led Zeppelin, and many other bands that were known for their guitar work.  Since the D is now a full half-step lower, you can play simple 2-finger power chords with ease-- just hold the 6th and 5th string on the same fret. This "drop D" tuning also allows for a deeper, darker sound, which many metal and alternative guitarists now prefer. Your whole band will be out of tune if you're not on the same page. The third fret on the top string, for example, is not a G if you're in Drop-D -- it is a half step lower -- a C#.

Summary:
Tune your top string down to D to get Drop-D tuning, allowing a deeper sound and easier power-chords. Make sure the rest of the band knows you're using a drop tuning.