it's a term used to name the music style of bands like Green Day, Blink 182, Good Charlotte, or Simple Plan.
This bands were influenced by the skatepunk bands like Bad Religion, Nofx, Pennywise, etc. They made the songs catchier so it would become more listenable to the mainstream audience therefore more popular. That's where the name pop-punk comes from.
Being mainstream music has caused the rejection of this music by other punkrockers who are against the mainstream and corporate music.
Nowadays pop-punk is influating other bands to create new styles of music like rock-pop or new hardcore-pop better known to the audience as emo music
Green Day, Blink 182, Sum 41, Good Charlotte, Simple Plan are the best examples of pop-punk