Drop B tuning
The Drop B tuning is mainly associated with metal genres. This tuning does two things to the guitar sound: it delivers much more bass and it gives possibilities to play chords like C sharp and B with open strings.
Here is how you tune the guitar: B F# B E G# C#. The drop B tunings deviate from the standard tuning on all six strings, but it helps you to know that the distance is three semi-tones on all strings except the 6th string (B), that is five semi-tones down from the standard E.
It's important that you turn the tuning pegs in a direction that decrease the string tension and not the opposite; otherwise, you could break the strings and possible damage the guitar.
As you can see from the diagrams, power chords with the bass on the lowest string can be played with one finger. The normal power chord shape is used when the root note are on other strings, as seen in the diagram example above that show A5.
Some other power chords in short notations:
C5: 111XXX / X688XX
D5: 333XXX / X81010XX
G5: 888XXX / X133XX
Some other chord possibilities
Here is a collection of other chords that can be used in the Drop B tuning.
As you may notice, you can to a high degree handle the key of B as the key of D on a standard tuned guitar. The only exception is the 6th string.
This was an introduction about chords in the Drop B tuning, see also Drop C tuning.