Drop B tuning is an alternate tuning for a six string guitar used by metal guitarists to achieve a deep and powerful sound. To get this low 7 string sound on a normal 6 string you need to do get yourself some really thick and heavy gage string like 13 through 56 or a 7 string set and just not use the 9.

In drop B tuning the strings are usually tuned (from low to high string) to B-F♯-B-E-G♯-C♯.
Some guitarists use the alternative variations on drop B where their strings are tuned to B-F♯-B-E-G♯-C♯, or even  B-G♭-B-E-A♭-D♭.

You can hear drop B used by Sepultura on their Roots album, by Slipknot and sometimes by Dimebag Darrell when he played with Damageplan.

This will change your string tension - so your guitar might need setup changes (check the intonation and the action, and maybe the string grooves at the nut).

http://www.youtube.com/watch?v=B3-lVCiFio4