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 here 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.
Play the following example and listen to the way the E (natural 3rd) and Eb (flattened 3rd determine whether the chord is C major or C minor.
In this example, both the 3rd and the 7th are necessary to indicate the chord types.