This is one of those words I'd heard in German, as a child, years and years before I was aware of its use in direct translation in English - ein Ohrwurm. The German word "Ohr" reminds me so strongly of "bohren", to drill or bore a hole - and these tunes really do seem to bore a hole into my head, and go round and round. There is a book, I forget which, in which they get discussed in some detail - like the fact that most folks find it's not a whole melody from one end to the other we tend to hear, but rather small snatches, to which we'll come round again and again. I also find it tends to be the last thing I was either listening to or playing, and the bit about it being only small excerpts has a curious effect with me - classically, I'll be listening to music in the car on the way to work, won't listen to the end when I get there because it's on a CD....and find my head picks up again on the next trip out, just where the CD had been. Overnight I'll fall asleep thinking of something catchy I'd heard earlier, and wake up still thinking about it.
People who go deaf, especially if profoundly deaf, get a weird version of this too - they'll hear pieces vividly, in their heads, sometimes, and for some it becomes a very persistent musical tinnitus. In that case though they think it's because the brain expects stimuli, and when it doesn't receive any in the relevant bit of cortex, it simply "invents" some, usually drawing on musical memory.
My Ohrwurm of the day is some Vivaldi, at the moment. At least until I put the CD player on. I'm not going to play, myself - me flu has turned into a hacking cough, and sucking back will trigger that