From Wikipedia: "Earworm, a loan translation of the German Ohrwurm, is a portion of a song or other music that repeats compulsively within one's mind, put colloquially as 'music being stuck in one's head.' "
According to statistics, 98% of us get earworms at one point or another. They apparently last longer in women, and annoy them more :-). I would have to say that I suffer from earworms more so because I'm always listening to music more intensively when trying to figure out chords, licks and lyrics for my guitar students. And I'd have to say that Taylor Swift tops my list of earworm-causing songs. Bad, you say? Actually, not at all.
If you're a songwriter, the earworm is your friend! You want parts of your song to stick in people's heads, the longer the better. Hooks are often a cause of earworms. Do you remember James Blunt's song "You're Beautiful"? That earworm drove me nuts for days after I worked it out for a student. Blunt repeats that phrase over and over in the chorus and it was a huge hit for him. No wonder!