Drop D Tuning is used by many grunge, metal and heavy rock bands, as it makes it really easy to get a heavy rock sound using only one finger to barre the chords! Drop D Tuning is probably the easiest alternative tuning for guitar as it involves retuning only one string - the sixth string. It simply changes the 6th string from and 'E' note to a 'D' note.
To tune your guitar to "Drop D Tuning", start of with standard tuning (E A D G B E) and simply tune the sixth string down a tone to D. It should sound the same as the fourth (D) string, but an octave lower. You can do this with most electronic guitar tuners, but make sure you are tuning down (loosening the string), as if you try to tune up to D then you will probably break the string!
To check your tuning by ear, simply fret the 6th string at the 12th fret (or play the harmonic if you know how) then it should sound exactly the same as the 4th string played open (D). Your whole guitar tuning is now D A D G B E - you are tuned to "dropped D".