@dadonred pretty much nailed it. Prechoruses link the verses and choruses and are usually either exactly the same or have slight variations in the lyrics. Bridges are distinct sections (sometimes playing off the verses and choruses and sometimes are a big change) and normally come after the 2nd chorus and either lead into the guitar solo or back to a chorus. Bridges are more or less the "C" section in a song when a song goes ABABCAB or whatever. It's really interesting to me how much I love prechoruses and I know it's because I grew up listening to metal. For some strange reason a lot of metal songs feature prechoruses while other rock and pop doesn't. I'd be interested to know where the idea for pc's came from as far as rock bands go.