First, in order to define what G7 chords are, we must first understand what a 7th chord is in the first place. Where a typical triad is composed of the tonic, the mediant, and the dominant scale degrees of the given root note, a tetrad extends this triad by a fourth degree. In the case of 7th chords, this extension includes the 7th degree of the scale, the leading tone, giving it a characteristically tense mood. This extension could just as easily have been a major 7th, with this leading tone augmented so that it is in fact the note right before the scale loops back round on itself and becomes the octave. Thus, a G7 chord is simply a G major triad extended with its corresponding leading tone: tonic (G), mediant (B), dominant (D) & leading tone (F).