You have got to know how to build chords to be a thinking musician. Knowing how to construct chords will allow you to seamlessly play with different chord shapes and inversions, as well as build guitar solos that are based upon chord tones. It is the key to unlocking practical music theory.
Chords are built off the notes of the major scale. Here is a chart of some commonly used major "extended" chords, arranged to show how they build up:
Major – 1 3 5
7 – 1 3 5 b7
Maj7 – 1 3 5 7
9 – 1 3 5 b7 9
Maj9 – 1 3 5 7 9
11 – 1 3 5 b7 9 11
Maj11 – 1 3 5 7 9 11
13 – 1 3 5 b7 9 11 13
Maj13 – 1 3 5 7 9 11 13
(For minor chords – replace the 3 with a b3. For example: Min11 is spelled 1 b3 5 7 9 11)
For extended chords, the tonality hinges on the seventh note (7 vs b7).
A common misunderstanding in chord building is what "maj" means. "Maj" refers to the seventh note in the chord. An Amaj7 has a 7 note in it (G#). An A7 has a b7 note (G).
The payoff to understanding the content in this lesson is being able to look at any chart and never be confused. Sure, you may not actually know the chord you are looking at, but with practice you can easily build it.
Start dissecting the chords you know. What notes are they made up of? What order are those notes in? Grab a book with some interesting chords, and practice constructing them.