pingouin is correct, but a little more needs to be said. Rhythm Changes is the name given to any song with the following progression:

| Bb    G7    | Cm7    F7   | Bb    G7  | Cm7    F7   |
| Fm7 Bb7 | Ebmaj7 Ab7 | Bb G7 | Cm7 F7 |
| Bb G7 | Cm7 F7 | Bb G7 | Cm7 F7 |
| Fm7 Bb7 | Ebmaj7 Ab7 | Cm7 F7 | Bb ||
| D7 | D7 | G7 | G7 |
| C7 | C7 | F7 | F7 ||
| Bb G7 | Cm7 F7 | Bb G7 | Cm7 F7 |
| Fm7 Bb7 | Ebmaj7 Ab7 | Cm7 F7 | Bb ||

Or any variation on these chords. Common variations include | Dm7 G7 | instead of | Bb G7 | (which is a standard substitution), or | Ebmaj7 Ebm7 | instead of |Ebmaj7 Ab7 |. Also, common substitutions are found over the bridge, most commonly ii-V instead of a dominant 7 chord (for example
| Am7 | D7| instead of | D7 | D7 |).

The I-VI-II-V (Bb - G7 - Cm7 - F7) is the hallmark of the rhythm changes, and gives them their unique sound.

Rhtthm changes are the second most popular chord progression in jazz music (the first being, of course, the blues). The first rhythm changes was I Got Rhythm by George Gershwin (hence the name), and the chord changes are so simple and elegant that many jazz artists took the chords and wrote new melodies to them. Among the most famous are Oleo, Anthropology and Moose the Mooche. But the one you (the reader of this node) probably knows the best is the theme from The Muppet Show. Yup, it is, indeed, rhythm changes. As are many other popular childrens songs (can you find any more?)