Power Chords

This is commonly used with many different types of music, especially rock.
Notice the pattern? You just simultaneously play the base note (2nd fret on the bottom E string in the first power chord) as well as the note that is 2 frets above it on the next string. The notes in the brackets are optional and should be played to your preference.
These are all valid ways of fingering a power chord. In the pictures the fingers not in use are curled towards the palm, and in the middle picture the little finger is resting against the ring finger, but this is just so that they're not blocking the view. You should keep them relaxed and ready to play the next notes instead. Please note that if possible the forefinger should also lightly touch the rest of the strings to dampen any unwanted sounds.