Or more accurately, snippets or pieces of chord progressiions that serve to give the piece continuity and let the music flow.
There are umpteen harmonic formulas most of which can ,and are , utilized in segments like bridges ,turnarounds, cadences etc.
A really good point of study is to learn a half dozen turnarounds you could use, to start a tune off for example. This allows the listener to ease into the head and if played properly with the last chords being a two-five-one
OF THE KEY your going to play the song in, you might actually play the entire intro in a totally different key but IT STILL SOUNDS GREAT!
O.K I digress but anyway if you learn various ways of playing even the standard formulas you will be way ahead in no time and will actually start hearing the very same in songs on the radio or wherever!