Gargoyles

I have a question concerning gargoyles.

A topic on which I don't believe I have ever asked a question in my life.

Anyway, so many of these churches have gargoyle images in them, staring down on the interior of the monstaries usually.

Why? What is the significance of the gargoyle? I assume there are people out there who have more knowledge on this than me. I know nothing on medieval Europe and my tolerance for the kind of contemporary writing that uses gargoyles and other such things is extremely low.