A chord is a combination of different notes played simultaneously. Some chords, such as power chords, are called chords but are not because they do not contain enough notes (power chords contain only two different notes). A true chord has at least three different notes. When a chord has exactly three notes, it is also called a triad.
Another term in this context is arpeggio. Arpeggios are chords, but the notes are played one after the other, not simultaneously.
Several chords played in succession are called a chord progression.
(It is highly recommended to read the introduction to intervals first.)
The table below contains the most common chords. With the help of the given formula (intervals / scale degrees that make up a chord), you can form the corresponding chord from any root note.
Let's apply this to the C major chord as an example. Every major chord has the formula 1 - 3 - 5. The root of a C major chord is, of course, C. On top of C we add the major third E (four semitones higher). To complete C major we add the perfect fifth G (seven semitones higher counted from C).
If we look at the chord diagram of C major, we see that the chord contains the necessary notes C, E and G. In addition to C, E and G, the octaves of C and E are included. This wouldn't be necessary to get a C major chord, but it's convenient because the guitar has more than three strings and the chord sounds much fuller this way.
|Major||1 - 3 - 5|
|Minor (m)||1 - b3 - 5|
|Augmented (aug)||1 - 3 - b6|
|Diminished (dim)||1 - b3 - b5|
|Suspended second (sus2)||1 - 2 - 5|
|Suspended fourth (sus4 or sus)||1 - 4 - 5|
|Major sixth (6)||1 - 3 - 5 - 6|
|Minor sixth (m6)||1 - b3 - 5 - 6|
|Major seventh (M7)||1 - 3 - 5 - 7|
|Minor seventh (m7)||1 - b3 - 5 - b7|
|(Dominant) Seventh (7)||1 - 3 - 5 - b7|
Returning to our C major chord, we learned that it consists of C (root), E (major third), and G (perfect fifth).
If you now play the C an octave higher, i.e. E - G - C instead of C - E - G, you still have a C major chord, but with a different voicing (it sounds different - try it out). E - G - C is called the first inversion of C major. G - C - E is the second inversion of C major, which again has a different voicing.
Other simple ways to change the voicing of a chord are to play two notes an octave higher, or to play one note plus the octave of the same note (the above C major chord has a special voicing because of the two added octaves).