Welcome to a short introduction of the theory behind chords. You will learn how a chord is built and what separates different groups of chords.
In the cases when we only have a letter, such as C, it’s a simple major chord. A major chord consists of three notes: 1st, 3rd and the 5th notes in the scale. In which scale, you may ask. In the C major scale we have:
C – D – E – F – G – A – B
So for a C major chord we can locate the 1st, 3rd and 5th as C, E and G.
Sometimes we find chords with names like Cm, Dm, Em and so on. The “m” stands for minor and in a minor chord we have three notes: 1st, 3rd minor and 5th. The notes in Cm are C, Eb (E flat) and G.
You may ask how it comes that we only have three notes in a C major chord when you have learned to strum five strings playing this chord:
C major chord
The reason is the instrument. The notes are ordered less symmetrical on the guitars fretboard opposed to the piano keyboard. Another reason is that musical context differs from piano to guitar: you cannot play the harmony with your left hand and the melody with your right hand simultaneously on the guitar. Therefore, we don’t need to worry about too many notes being played at the same time.
As we can see in the diagram above, there's five notes played together in a C major chord: C, E, G (open string), C and E (open string). It's the most practical way to play the open C chord. In chords, we often find numbers after the letters: C5, C7, and C9 to list a few. In a C5 chord (also called power chord), we only have two notes: the 1st and the 5th. That's pretty simple. The C5, therefore, consist of C and G.
In the case of the seven, things are little messier. First of all: there’s C7, Cm7 and Cmaj7. C7 is called the dominant 7th. A C7 consists of the C major chord plus the flat 7th. In a C scale the flat 7th is Bb (B flat). In a Cm7 chord we add the flat 7th to a C minor chord. Finally, in a Cmaj7 chord we add the major 7th to a C major chord.
On this site you find lots of other chords like sus chords and slash chords. These chords have special sections and are explained together with its theory there as well. Moreover, you will get supplementary knowledge by reading What is a guitar chord?.