Drop C tuning
The drop C tuning is mainly used in heavy metal and could give you a real heavy and bassy sound. A useful technique to implement is palm muting.
Here is how you should tune the guitar: C G C F A D. From standard tuning you tune down the lowest string two whole steps and then one whole step for the rest (see the picture).
It is very important that you turn the tuning pegs in a direction that decrease the string tension and not increase them, otherwise the is a great risk you will break the strings and possible damage the guitar.
The chords above are as noticeable very simple. You could see the similarities of power chords (in both cases the chords are moveable and only include a few and the most bassy strings) and you should also in a high degree used them in a similar way.
An interesting thing is that you can play barre chord as normal, despite the changed tuning, and they will sound good. In some cases it will also sound good and slight different than normal with open chords Since five of six strings have been change with the same proportions (one tone down) it will still sound pretty much as usual, but slight different. Try for yourself and see.
This was an introduction about chords in drop C tuning, see also open D tuning and open G tuning