I thought maybe this would be useful for some here. This is a quick and dirty introduction to building chords. If you know this stuff then you can play most chord someone names off (assuming you have a moderate knowledge of the notes on the fretboard).
The Major Scale
The first thing you need to pick up is the major scale. One way to construct the major scale is to use the WWHWWWH step by step construction. I'll show you how this works. Having a good knowledge of your intervals will help as well.
If the first note is C then the next note is a Whole step up from that, D. Another Whole step is E, Half a step is F...you get the picture...in the end you have CDEFGABC. If you want to build any other scale you just work from the note it is named after and build it up.
The notes in scales are numbered in 'degrees'. The first octave is pretty straight forward (1, 2, 3, 4, 5, 6, 7, 8) but after that it gets a little odd. You keep counting up from 8 in the next octave but the 1, 3, 5, and 7 don't get changed. So the whole thing ends up looking like: 1, 2, 3, 4, 5, 6, 7, 1, 9, 3, 11, 5, 13, 7.
On the guitar you often play inverted voicings where the high numbers are lower in octave than the low numbers. That is why often times it is good to think of the 2 and 9 as being the same note when technically they are an octave appart.
The C major scale and its degrees are:
There is no such thing as a chord that does not have the 1, 3, and 5 scale degrees in it. That is the technical definition of a chord. On the other hand, we as guitar players often leave one or more of these out. But for the sake of this 'lesson' a chord has a 1, 3, and a 5. There are 4 types of chords that come out of this: Major, Minor, Diminished, and Augmented. If you think back to what your major scale is and what these numbers mean it becomes very easy to construct all 4 of these types. A Major is the 1, 3, and the 5. A Minor has a flatted 3. A Diminished has both a flatted 3 and a flatted 5. The augmented (which doesn't technically fit into any key) has a normal 3 but a sharped 5.
There are "chords" without the 3. These are the suspended 4th and 2nd. They replace either the 4th or the 2nd, respectively, with the 3rd. There are also power chords or 5ths that leave out the 3 entirely. You can usually choose to interpret them as major or minor on a whim.
The confusing stuff
When you get into chords with more than the 3 chordal tones you start getting confused. This is caused by the 7th. You would think that a 7th chord has a 1, 3, 5, and a 7 from the major scale but this is not so. When people say something like C7 they mean C dominant 7 and the dominant 7th degree is flatted. This is the 5th chord in the 12 bar blues pattern and fits the Mixolydian mode.
Any chord that has a number after it technically contains this dominant 7th unless otherwise specified. So for instance C9 is really C7 with a 9th degree note added to it; C11 is C9 plus an 11; C13 is C11 with an added 13th. Of course it became impossible to play these chords quite a while back, so as guitarists you remove tones; sometimes even the root is ommited (let the bass player do it :P)
When someone wishes to play one of these chords with the standard 7th from the major scale they explicitly state the 7th is major. This is where you get CMaj7; it is a C7 with the 7th from the major scale instead of the dominant 7th. CMaj9 is a C9 with that same major 7th instead of the dominant. There is also the subdominant or double flatted 7th but that is beyond my level.
The way wack chords
That is it for 'standard ' chords were you don't modify any degree. However, that far from spells out any chord. The rest of the chords are created by altering notes in the above scenarios. For instance, C7b9 is a C7 with a b9 added, or a C9 with its 9th flatted. You can do that with any degree and you can get quite rediqulous with it (but really eventually you should think about calling the chord by a different root note). There are also a few notational things to consider. A + after the name of the chord means it is augmented (#5). Some common ones are C+11 which means C11 with a #5. A diminished chord has a degree symbol by it or is labeled Cdim in standard text.
Some composers also label + or - instead of # or b. Altered degrees are put in paratheses usually when this is done: C7(-9) is the same as C7b9. There is of course much more to this than I am going to, or even can, write about here.
When you see something like "add" in the name of the chord it means that we are just adding that degree. So for instance Cadd9 is a stardard C chord with a 9th degree note added, the 7 is not included.
Well now you are armed with some of the tools for building chords. The chord is named after its root note so start there. Build a major scale from the root note and circle the degrees that are in the chord. Remember that 1, 3, and 5 are almost always there. Then alter the notes that are altered, like the 7th probably. Now find those notes on the fretboard. There you go. The trick is to see these patterns on the fretboard immediately and be able to construct chords on the spot. I am not there yet. Lets try a few:
F Major: F G A Bb C D E F
Degrees included: 1, 3, 5, 7 Notes: F A C E
Alterations: b5 Cb -> F A Cb E (E is left alone because this is 'maj7')
G# Major: G# A# B# C# D# E# F## G# (fun)
Degrees included: 1, 3, 5, 11 Notes: G# B# D# C#
Tough one: B9 sus4
B Maj: B C# D# E F# G# A# B
Included: 1, 4, 5, 7, 9 Notes: B E F# A# C#
Alterations: b7 A -> B E F# A C#
E Maj: E F# G# A B C# D# E
Included: 1, 3, 5, 6 Notes: E G# B C#
You might want to do your alterations first and spell your scale with the altered notes. Most chords fit into one of the 8 modes. Any chord that is true to a key will fit in a mode, but not all chords are true to any key. For instance, there is no mode that has a #5 in it which means there is no key that any augmented chord is actually a member of.
Hopefully that helps. There is a lot on the net about this stuff including: