Barre chords are essentially open-string chord shapes that can be formed at different points on the fingerboard. To form a barre chord:
1. The first finger is stretched across the width of the fingerboard.
E[--2-----
B[--2-----
G[--2-----
D[--2-----
A[--2-----
E[--2-----
Kind of like putting a capo on a certain fret.
2. The remaining three fingers are used to form the chord shape. An E Barre Chord thats the shape of an E chord, but is actually F#:
E[--2-----
B[--2-----
G[--3-----
D[--4-----
A[--4-----
E[--2-----
So now, you are forming an E chord shape, but since it has moved up the fingerboard, you are actually playing F#. Essentially, the first finger acts as the nut, or zero fret. The great thing about barre chords are that they allow open-string chord shapes to be played in any key. The most commonly used barre chords are E Shaped (like the exercise above) and A Shape. Less common are those formed around the open C and G chords. They are possible, but much trickier to pull off. Barre chords are also sometimes known as "slash" chords.
Why do barre chords work? Barre chords work because the first finger acts as a repositioned nut from which open string chord shapes can be built. In practice, this is more complex since you no longer have a first finger to fret individual notes. Some guitarist like the way the open string chord shapes sound. Others appretiate the ease with which chord changes can be made. Therefore, some players choose to, instead of forming barre chords, they place a CAPO on a fret instead. The capo works by fitting over the strings of the fingerboard and pressing down, using a clamp on the back, acting just as the first finger of a barre chord would word, but now, you have the first finger back, being able to fret individual notes. If a capo were fitted on the fifth fret, and a standard E Major chord shape were formed, the resulting chord would be A Major.
The E Shaped Barre.
As we worked on above, the E Shaped barre chord works by having the first finger pressed against all the strings of a certain fret. After this is done, the remaining three fingers are free to form an E Major chord shape. The E Major Chord Shape:
E[--0-----
B[--0-----
G[--1-----
D[--2-----
A[--2-----
E[--0-----
If a barre is formed on the fourth fret, and an E Major chord is formed, the resulting chord is G#. The correct chord name for an E Shaped barre chord can always be found by finding the barred note you are fretting on the sixth string.
The A Shaped Barre.
The A Shaped barre works in the same way as an E Shaped barre, but you only need to barre the first five strings. The sixth string is optional, because the ROOT will always be on the fifth string. A regular A Major chord looks like this:
E[--0-----
B[--2-----
G[--2-----
D[--2-----
A[--0-----
E[-(0)----
Again, the sixth string is optional, and even though its musically correct, in some cases it just makes the chord sound wrong. Most players like the lessened strength it takes to hold down one less string in an A Shaped barre chord anyway, so this should'nt be a problem. So, forming an A Shaped barre on the second fret of the fifth string, will result in a B Major Chord.
E[--2-----
B[--4-----
G[--4-----
D[--4-----
A[--2-----
E[-(2)----
I find the chord sounds better when the sixth string is not played.
Making barre chords easier.
It's good to have a low action on your guitar if you use barre chords frequently. The action of a guitar refers to the hight from the string to the top of the fingerboard. This can easily be adjusted to your likings at a guitar shop, and I would only reccomend doing it yourself if your confident in your abilities. Well, thats about it. If you wish to know about the C and G Shaped barres, then, please, by all means PM me in the forums, and I'll send you some information on it. Because they are very rarely used in popular playing, I have not included them in the lesson. Well, I hope this can help with your barre chord needs.
/ my profile 
