Intervals

To understand chords, you need to understand "intervals" first. An interval is the auditive distance between two notes.

The shortest interval is a "semitone", also called a "minor second" (m2), such as the distance from C to C#, or from E to F.

[Strictly speaking the shortest interval is two notes played at unison, but here were are not so much interested in notes that sound the same, but in the sound of two different notes.]

Two semitones make a whole tone, or, "major second" (M2) interval, such as the distance from C to D, or from G to A. When played together, it produces an unpleasant/dissonant sound.

A "third" is the distance between C to E. There are two types of "thirds": major third, and minor third. Thirds produce pleasant sounds.

A "major third" (M3) is four semitones, such as from C to E, or from F to A. This is the basis of all major chords.

A "minor third" (m3) is three semitones, such as from C to Eb, or from E to G, or from A to C. This is the basis of all minor chords.

A "perfect fourth" (P4) is five semitones, such as from C to F. The sound of a perfect fourth is pleasant.

A "perfect fifth" (P5) is seven semitones, such as from C to G. The perfect fifth also produces a pleasant sound.

A "diminished fifth" (d5) is six semitones, such as from C to Gb/F#. A diminished produces a tense/dissonant sound.

There are also "minor (m6) and major (M6) sixths", "minor (m7) and major (M7) sevenths", and other, intervals.

The following image illustrates the main intervals, using C as reference: