Drop D tuning (aka drop tuning or slack key) is one of the easiest alternative guitar tunings because only the lowest string is different from the standard guitar tuning. The drop D tuning is very popular in metal because it makes it a lot easier to play power chords. It is also popular in other guitar styles like rock, folk and country.
To D tune your guitar, all we need to do is retune the 6th string from E to D:
You can do this with a guitar tuner or by ear:
Tune your guitar in standard tuning.
Play the 4th (D string) and 6th string together and turn the tuning key of the E string counter clockwise until you are 1 step lower. Then go a bit below the pitch and tune up to match the 4th string (1 octave difference). It's important to go a bit below the pitch and tune up, it prevents the string from slipping out of tune.
Tune the other strings again, it might be that they are out of tune because of the drop d tuning.
In drop D tuning it's very easy to play power chords. Just bar your index fingers across the 3 lowest strings and you have got a power chord. This is for example an F power chord (F5):
Drop d tuning also makes it possible to add a nice deep bass to D chords:
Other common chords that have their root on the E string change as well. E chords are different in drop d tuning compared to standard tuning:
Bar chords change as well, here's an F bar chord for example:
An open G is difficult to play in drop D tuning.
Here's a list of songs in drop d tuning:
Dear Prudence - The Beatles
The Chain - Fleetwood Mac
Ohio - Neil Young
Harvest Moon - Neil Young
All Apologies - Nirvana
Something in the Way - Nirvana
Moby Dick - Led Zeppelin
Kashmir - Led Zeppelin
Higher - Creed
Everlong - Foo Fighters
Black Balloon - Goo Goo Dolls
I Miss You - Incubus
Cold - Crossfade
Grand Theft Autumn - Fallout Boy
Meant to Live - Switchfoot
So Cold - Breaking Benjamin
Sludge Factory - Alice in Chains
Vermilion Part 2 - Slipknot
Grace - Jeff Buckley
Black Hole Sun - Soundgarden
The End - The Doors
I'll give you some examples of songs with an open d tuning. Here are the guitar tabs for All Apologies by Nirvana. Play the video if you don't remember how the song goes. Kurt Cobain's guitar is tuned a half step lower compared to standard drop d tuning, so don't be alarmed if you sound out of tune with the video.
Here's another great song with drop d tuning, Moby Dick by Led Zeppelin (with video):