In any chord, there are certain notes which tell you exactly what the chord is. In major and minor triads it is the 3rd (the 5th is the same in both chords) and in seventh chords, it is the 3rd and the 7th. These notes are called guide tones. Shown below are various chords built on a C root note, with the guide tones indicated. Notice that these are the notes which change from one chord type to another, while the root and the 5th remain the same.
In the following example, the piano plays the notes C and G, while the sax alternates between E and Eb to indicate a C major chord and then a C minor chord.
In this example, both the 3rd and the 7th are used as guide tones to indicate the chord types.