Pop-punk in my opinion:

1. A rock group that USED to be punk, and now has fallen from their glory. (Green Day, maybe?)

2. A band or artist that try to act extremely badass, but still is still just pop with guitars. (Avril Lavigne... or modern Green Day trying to be all political?)

3. Bands that are a bit heavier than bands like... say... Switchfoot, but are still poppish. (Sum 41)

But that's just me.