Sometimes people feel guilty when they eat cake. For example, if you are on a diet, eating sweet things is not good, thus the feeling of guilt when you indulge in pizza or cake. So we call that a guilty pleasure. Guilty pleasures can be all sorts of things, depending on the character of the person. A virgin would call sex a guilty pleasure, most of us just call it pleasure. Some people think a glass of Baileys is a guilty pleasure, I just drink the stuff.

Guilt(y) pleasure is an activity that you believe is not usually good for one's self, but you desire the activity more and act on it. For example: Instead of buying a healthy snack, you buy a less healthy, but delicious snack because you crave more.

It's something you think you shouldn't enjoy but you do enjoy... you could have your own reasons to feel like that or you can be concern about how people would react if they know you are doing it . Ice cream example is great... Imagine you run to the freezer every night at 3 in the morning to have two full spoons of your favorite flavor (which you hide there), even if you think you need to lose 2 or 3 pounds... now, that would be your GUILTY PLEASURE !

In plain English, a guilty pleasure is something that you enjoy doing but is something you view as not good for you.

Eating lots of chocolate or ice cream even though you are on a diet.