Barre chords are played by pressing a finger across multiple strings on the fretboard. In most cases, the barring finger acts as a capo, allowing the remaining fingers to fret various chord shapes.

Because no open strings are involved, barre chords can be moved to various positions on the neck, without changing their structure. This is what is refered to as a movable shape.

Barre chords are based off of open chord shapes. The 2 most common types are E form (6th string root), and A form (5th string root).

Form:

A form (based off of an open A maj7 shape)

Voicing:

R,5,7,3,5

* R = Root