Top 40 is a radio format based on frequent repetition of songs from a constantly-updated list of the forty best-selling singles. The term is also used to refer to the actual list of hit songs, and, by extension, to refer to pop music in general (as defined by wordiq.com).
There is no particular style of music that can be clearly identified as pop. This is because pop music is always changing. It can be identified as the most popular mainstream music of the time. Since the 1950's, this has been the music that has evolved in the wake of the rock and roll revolution of the 1950's.
The most consistent component of pop music has been the concept of the pop song. Most pop songs range in length from approximately 2 minutes to 5 minutes, and they frequently include a verse-chorus-verse-bridge-chorus structure or some close variant. Longer compositions are rare in pop music.
Pop music has been and continues to be a melting pot of styles. The genres most frequently drawn from are hip hop, R&B, rock, and country. However, influences have been pulled from music as widely variant as classical and African drums (Bill Lamb, About.com Guide).
Beach music, also known as Carolina beach music, is a regional genre which developed from various musical styles of the forties, fifties and sixties. These styles ranged from big band swing instrumentals to the more raucous sounds of blues/jump blues, jazz, doo-wop, boogie, rhythm and blues, reggae, rockabilly and old-time rock and roll. Beach music is closely associated with the style of swing dance known as the shag, or the Carolina shag, which is also the official state dance of both North Carolina and South Carolina. Recordings with a 4/4 "blues shuffle" rhythmic structure and moderate-to-fast tempo are the most popular music for the shag, and the vast majority of the music in this genre fits that description (Wikipedia).