Pop is imo any song with a good melody and "hooky" (sing-along) chorus refrain. Pop songs have traditionally also followed a classic structure (though they don’t necessarily have to): Intro/1st verse/chorus/2nd verse/chorus/bridge (called middle 8 in England)/verse/last chorus/outro. The bridge/middle 8 is employed less frequently now, often ignored by Rock Bands that don’t have a superior songwriter (most ;-). During the Psychedelic period (starting in 1966), Lennon & McCartney started "messing" with the Pop song structure. They---being more-talented songwriters than most in Rock Bands---could pull it off; most didn’t, leading to some of the worst "songs" I’ve ever heard.
A sterling example of a Pop song is "When You Walk In The Room", written by Jackie DeShannon and released by her as a single in 1963. The Searchers covered it in ’64, and was the first time I heard this majestic masterpiece. Springsteen has included the song in his live show, though he as always bludgeons the song to death. Agnetha Faltskog of ABBA (now THERE’S a fantastic Pop Group!) does a great version, found on her My Coloring Book album.
Then there is "What becomes Of The Brokenhearted", originally done by Jimmy Ruffin (Joan Osborne stole the show with her version at the tribute concert for the Motown house band). Written by William Weatherspoon, Paul Riser, and James Dean (not the actor, of course), it is a breathtakingly great Pop song.
Another at the very top of the form is "God Only Knows", written by Brian Wilson (music) and Tony Asher (lyrics), and first recorded by The Beach Boys and included on their Pet Sounds album. Paul McCartney, a pretty fair songwriter himself, has said he considers it the best song he has ever heard. Another stunner by Wilson (music and lyrics) is "Til I Die"---melancholy beauty rarely heard (found on the 1971 album Surf’s Up).
Is Tom Petty Pop? Well, Tom Petty & The Heartbreakers are a Rock Band, not a Pop Group. The Mamas & Papas were a Pop Group, as were ABBA, The Carpenters, and most other Vocal Groups. But a Rock Group can make a Pop record, and many of the best do. Cheap Trick are very Poppy ("Surrender"), but are obviously a Rock Group. The Ramones were also very Poppy (and proud of it. They were thrilled to be produced by Phil Spector---that is, until he locked them in his castle and wouldn’t let them leave), but no one’s going to mistake them for The Association.
A Rock Band performing Poppy material is commonly referred to as Pop-Rock in music criticism, a category that includes a lot of people. Guys like Emitt Rhodes (easily McCartney’s equal in songwriting and singing, and a better drummer ;-), Squeeze, Marshall Crenshaw, Nick Lowe, hundreds of others. Pop songs performed in the Rock style. Though they were about as far from being a Pop Group as is possible, "The Weight" by The Band is a perfect Pop song.