This is what we found on Wikipedia regarding the 19-Twenty:

Pop music is a genre of popular music that originated in its modern form during the mid-1950s in the United States and the United Kingdom. During the 1950s and 1960s, pop music encompassed rock and roll and the youth-oriented styles it influenced. Rock and pop music remained roughly synonymous until the late 1960s, after which pop became associated with music that was more commercial, ephemeral, and accessible.

Identifying factors of pop music usually include repeated choruses and hooks, short to medium-length songs written in a basic format (often the verse–chorus structure), and rhythms or tempos that can be easily danced to. Much of pop music also borrows elements from other styles such as rock, hip hop, urban, dance, Latin, and country.

The terms popular music and pop music are often used interchangeably, although the former more accurately describes all music that is targeted for mass appeal (compare art music) and includes many disparate styles. Although much of the music that appears on record charts is considered to be pop music, the genre is distinguished from chart music.