These are the chords that are played with 3 notes that we will call "fundamental note" (which gives the chord its name), "third" (located at a distance of a third above the fundamental) and "fifth" (located at a distance of fifth above the fundamental).
It depends on what kind of third and fifth intervals the chord has, we will classify them into four types: major, minor, Augmented or diminished.
The major chords have the third major and the fifth major(1-3-5). They are indicated by a capital letter.
The minor chords have the third minor and the fifth perfect (1-b3-5). They are indicated by a capital letter followed by "m" or the "-".
(By saying b3, it means that on the corresponding major scale of the chord, the third grade is lowered half a tone.
The augmented chords have the third major and the fifth augmented (1-3-#5). They are indicated by a capital letter followed by the "+" or the "aug" suffix..
The diminished chords have the third minor and the fifth diminished (1-b3-b5). They are indicated by a capital letter followed by the symbol "º" or the suffix "dim".
When playong them we can put any note that are part of the chord as as the lowest note.. When the note that names the chord is the lowest, the chord is in fundamental position. If the lowest note is the third, it is in first inversion, and if the fifth is the lowest note, the chord is in second inversion .