Hi Rob,
It sounds like what you want to do is create a Cycle. To do this:
1) Set the left locator at the beginning of the section you want to loop
2) Set the right locator at the end of the section you want to loop
3) Click the "Cycle" button on the Transport (looks like two arrows circling around each other).
You'll know you're successful if the area of the bar ruler in between the two locators turns green.
Once you have created your Cycle, when you hit Play, it will play endlessly between the Cycle markers, and when you hit Record, it will record endlessly between the Cycle markers. You can Toggle the Cycle by clicking the Cycle button on the Transport.
There are also loop commands (default: L) for individual regions/tracks, but it sounds like you want to loop a whole section of the song.
Hope that helps!
Orren