No, it's not catching - at least I don't think so. An ear worm is that little tune, or jingle, or snatch of music that keeps playing in your head. Don't pretend that has never happened to you, because apparently this affects almost 100% of us at one time or another.
It's not anything bad per se, but of course if you are the kind of person for whom little things build up to be an intensely annoying thing, then that would be unfortunate. In my case, I usually treat them as a pleasant little diversion - a sort of little soundtrack to my life.
For me, more often than not, these ear worms seem to take the form of snippets of music from my young adulthood - 70's music, predominantly. Right now, the "ear worm of the day" is the chorus to the song "The Ballad Of Cherry Flavar" (yes, it's spelled like that) by The Marmalade. I heard it the other night and it got wedged in there. I *think* it was the "B" side to a single of "Sarah", another Marmalade song I still like.