What distinguishes pork sausage from ground pork? Let me preface this by saying that I am basically a complete novice when it comes to cooking. So I apologize if this is a silly question.

I sometimes like to cook things using ground pork sausage, but much of the time it's nearly impossible to find in the local grocery stores. However, they always seem to have ground pork, which I've considered using as a substitute. I know it wouldn't cause my kitchen to explode or anything like that ;-) but I'm curious, what's the difference? What exactly am I putting in my food by using the sausage that I'd miss out on if I used regular ground pork?
Ground pork is simply that, pork. Pork sausage is ground pork that has been seasoned.

You can substitute, but you'll have to bring your own seasoning.