Cycle of fourths and Cycle of fifths are interchangeable, in that you can go up the cycle of fourths by going down the cycle of fifths, and vice versa. Classical musicians tend to refer to it as the circle of fourths, jazz and rock players tend to refer to it as the cycle of fifths. A good example of a progression based on the cycle of fiths (as I call it) is the tune made famous by Hendrix; Hey Joe. The chords are all major chords going clockwise around the circle starting on C and ending on E. So the chords are C, G, D, A, E. As you can hear, although these chords are not all in the same key, the progression sounds really smooth. This is because there is only one note's difference from one chord to another in the cycle of fifths. The smoothest way of changing from one key to another is to follow the cycle which is why it is so common for a pop song to go from C to G.