When I drive to town I almost always turn left on Oak, then right on Pine past the high school, then swing onto the freeway for 3 miles and get off on the first exit by Elmers Pancake House and then swing up Main St to the Post Office and over on Magnolia to Quick Print. Is that the only way I could get there? Of course not. There are probably a dozen or more ways I could go, but like most people, I am a creature of habit, and so day after day, year after year, I drive the very same way.
Chords are like that too. There are an infinite number of ways chords could progress, but if you toss all the songs ever written into a giant computer and have it spit out the most common chord progressions, you'll find that the top 5 or 6 progressions are used perhaps 80 or 85% of the time.
Why is that? Because songs are composed or made up on the spot by people -- not some music machine. And people take familiar paths, just like I drive the same way to town day after day.
Chord progressions are based on a series of chord changes, and these changes form the basis for the melody to be formed. Chord progressions are the harmonic backbone of a song, and they often dictate the song's tone and mood. Modern music tends to frequently base chord progressions out of the first, fourth, and fifth degrees of the scale; in C major, this would be C, F, and G. Of course, these chord progressions can be varied in a number of ways (chord substitution allows heavily for that), but these basic chord progressions tend to be the framework for a decent portion of modern music -- especially rock and pop.
Some of the most used chord progressions are: I, V7, I; I, IV, V7, I; I, vi, ii, V7. These progressions happen over and over and over and over again in literally thousands of song. For further information please check out Familiar Chord Progressions.
