BAR (OR BARRE) CHORDS

Bar chords are chords where the player uses a finger (usually the index finger) to bar the fretboard (ie fret more than one string) on top of which a chord shape is built using the other fingers similar to the shapes used for open chords. 

In fact most bar chords are closely related to their open counterparts, but with the index finger creating a moveable top nut. 

It is indeed this movability that makes bar chords so attractive.  A few shapes can be learned that can then be used all over the fretboard.