Most music in a major key has a bright sound that people often describe as cheerful, inspiring, exciting, strong, or happy.
Music that is in a minor key is sometimes described as sounding more solemn, sad, mysterious, or ominous than music that is in a major key.
This is called the Tonality of a chord (whether it is major or minor).
If we wanted to enhance the tonality of a chord i.e. make it sound more major or more minor, we can repeat the number of major or minor thirds in the chord, and omit some of the other notes.
For example, we could take a C minor chord and re-finger it like this:
Note that the chord now contains two minor thirds (Eb) and only one root. This will increase the 'minor' sound of the chord. We can do a similar thing with the root, to increase the root sound, or the fifth, to increase the dominance of the chord.
This demonstrates the importance of knowing where each note lies on the fingerboard.
In any key, the dominant note is always the 5th note of the scale (also called the perfect 5th). So, in the key of A, the dominant would be E.
A major or dominant chord built on this note (for example E or E7), will produce the strongest and most definite resolution to the root chord (A).
The sub-dominant note is the 4th note of the scale. This also has a strong resolution to the root, but not quite as strong as the dominant.
Together, the chords based on the root, the 4th and the 5th notes of the scale are referred to as the primary chords for that particular key. These chords are all major.
All other (minor) chords in that key are secondary chords (ie. chords based on the 2nd, 3rd, and 6th notes of the scale).