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Chords (Part I) introduced triads, explaining how they can be extended by adding extra notes which give your chords more colour.
In principle, you can add any scale tone to a triad and each scale tone will add its own unique colour to the triad.  However, in order to retain the musical nature of the chord, this tends to be done using certain patterns for certain chords in a Key.  However, never be afraid to use them in a random experimental way, you never know what you might discover.  One very common chord extension is the seventh which is explained in detail in this post.
To start with, I have to say when I started learning chords I found the notation for seventh chords really confusing.  It didn’t need to be, so hopefully the explanation below will make it clear for you from the start.
There are five main types of seventh chord, 2 types for the major triad, 2 types for the diminished triad but only one type for the minor triad.
Dominant Seventh Chords are a major triad and a minor seventh.  The chord is denoted by a ‘7’.  So for C7, the C major triad is C E G and the minor seventh is Bb.  C7 = C E G Bb
Major Seventh Chords are a major triad and a major seventh.  The chord is denoted by a ‘M’ and ‘7’ or ‘Maj7’.  So for C Maj7, the C major triad is C E G and the major seventh is B.  C Maj7 = C E G B
Minor Seventh Chords are a minor triad and a minor seventh.  The chord is denoted by a ‘m’ and ‘7’ or ‘min7’.  So for C min7, the C minor triad is C Eb G and the minor seventh is  Bb.  C min7 = C Eb G Bb
Half-Diminished Seventh Chords are a diminished triad and a minor seventh.  The chord is denoted by a ‘ø’ and ‘7’.  So for a C ø7, the diminished triad is C Eb Gb and the minor seventh is Bb.  C ø7 = C Eb Gb Bb
Diminished Seventh Chords are a diminished triad and a diminished seventh.  The chord is denoted by a ‘o’ and ‘7’.  So for a C o7, the diminished triad is C Eb Gb and the diminshed seventh is Bbb.  C o7 = C Eb Gb A
In any given Key, you tend to use the following type of seventh chord:
I – maj7
ii – min7
iii – min7
IV – maj7
V – 7
vi -min7
vii –  ø7
You really do need to play around with these chords to hear what difference the seventh makes to the chord sound.  For instance, some styles tend to restrict their use to the V chord, other styles may use them for each chord in a progression.
In addition to sevenths, there are a number of other frequently used extended chords which use sixths, ninths, elevenths etc and these are shown for each chord in the Key of C in the table below.  These can be transposed into any Key using the same formula given in the table.  Bear in mind there are a large number of combinations of other intervals typically used in jazz and classical music which are not shown here.

Ther are a few points to note:

• Sus2 and Sus4 chords replace the 3rd of the major triad with the second and fourth note of the Key respectively;

• Ninth, Eleventh and Thirteenth notes are added an octave higher than the triad notes but the chord also includes the 7 or b7 note for major and minor chord respectively;

• Sometimes eleventh chords are also written to also include the 9th note;

• Sometimes thirteenth chords are also written to include the 9th and 11th notes (although how you’re supposed to play a 7 note chord I don’t know);

• For some guitar voicings you don’t play all of the notes.

These theory posts are really only an introduction and barely scratch the surface. Hopefully they explain the basics and encourage you to explore and experiment further.