Essentially Americanized mayo. Cheaply produced and processed in massive quantities with lots of added sugar. Personally I find it disgusting (why would you want something savory so sweet?!) But my American grandfather loves it. If you would like to imagine it's flavor, imagine mixing vanilla ice cream with normal mayo.

It's one of those American staples that is a cheaper, more readily available, version of European staples that the American public continues to love and eat out of familiarity. American chocolate is similar, though more popular amongst younger Americans. I'd say miracle whip is less popular because quality mayo is now also readily available and very few prefer the cheap imitation. Quality chocolate is still much harder to find thus American chocolate remains popular.