Pop is all about the killer hook, the sing-along chorus, and the beat that gets you out of your seat. It’s all about the heart-tugging lyric and the soaring melody you can’t get out of your head. But more than anything, pop is the sound that brings millions of people together immediately. Since pop music tends to reinvent itself on the whims of a trend and revel in of-the-moment relevance, it's also a mirror of the times in which it was made. In the '40s, pop was defined by swinging jazz and snappy crooners. In the 21st century, pop has meant everything from cutting-edge electronic dance music to heartfelt tunes from country divas. The pop era starts with the rock ’n’ roll revolution of the late ’50s: the first sound aimed primarily at teenagers hungry for thrills, immediacy, and a booming backbeat. But after that? Well, the weird and wild history of pop music is a roller coaster that makes a twisting line between Beatlemania and "Gangnam Style."

It’s been a beautiful mess from the beginning. In the ’60s, pop was Phil Spector’s Wall of Sound–backed girl groups like The Ronettes, subtly subversive Motown-style soul, the sun-kissed surf rock of The Beach Boys, and countless catchy novelty rock hits. The ’70s brought bouncy, genre-bending Europop, the smooth sounds of soft-rockers, and the all-conquering beat of disco. By the ‘80s, pop had gotten seriously ambitious, with stadium-filling superstars like Madonna and Michael Jackson embracing funk, disco, synth-pop, rock, hip-hop, and more. That was also the era when country music and R&B truly became forces in pop, along with the more mellow sound known as adult contemporary. The ’90s will perhaps always be best remembered for the explosion of boy bands and girl groups that introduced teen icons like The Backstreet Boys and The Spice Girls, while the new millennium has given us a slickly futuristic dance-pop sound spearheaded by artists like Katy Perry and Rihanna. But pop ultimately remains too broad to be reduced to one trend or another. It’s where bubblegum fun meets brilliant sonic innovation, and where showbiz glitz mingles with personal expression. At least that’s what it is today—it wouldn’t be pop if it were the same thing tomorrow.