what they are
Simply put, a chord is any combined two strings or frets together to make a noise, but if you all ready guessed, two random notes combined could sound terrible. For E standard, there are patterns for these chords. There could be chords, power chords, abnormal chords and octave chords.
POWER chords
Power chords are probably the easiest chords to play because the only require two strings and two frets to play. To play a power chord you must have one finger on a fret and another finger on the string lower than your first finger and two frets higher than the first finger. If you are confused, look at the pictures below. For a better tone with your power chords you can continue with a third finger below the second, on the same fret.
Drop D Power Chords and Bar chords
when in Drop D tuning, it is easier to play these power chords. In Drop D, power chords are played with different strings on the same fret when playing the fattest strings. this called barring or bar chords and could be played in any tuning. because only one string is dropped a step power chords in this tuning could be played normal if excluding the fattest string or the D string.
normal chords
normal chords really don't have an order to each one of them, so they just have to be memorized and practiced. They are notes on frets that have no order.(speaking in tabs, anyway.) Just look at these charts and practice from them.
Octave chords
octave chords are essentially power chords that "skip" a string. An octave chord is played over a length of three or more strings with the "middle" strings muted.