When I was teaching biology, I came across a few children's science books that described fungus as "plants that get their energy from decaying matter." My nerdiness flared at this description, and I wanted to throw the book on the floor, yelling, "No! They are NOT PLANTS!" But I restrained myself and just made sure none of my students read this horribly mistaken description.
A quick note about moldy food: If you accidentally eat a piece of bread with a few moldy spots, it's not the end of the world. I've done it a few times, when I was apparently too busy to look at my food. Weird. But what you should avoid doing around moldy food is taking a big whiff of it. The spores (fungus babies) that mold releases can be a bother to your respiratory system. So please don't open a bag of moldy bread and stick your face while you take a deep breath. It won't smell good, and your lungs won't be happy about it.
Fungus has it's own kingdom (like the animal kingdom), and it's staggering how many different kinds of fungus there are. There are single-celled fungi, such as yeast. There are bizarre varieties that sound like something out of a science fiction novel. There are multicellular varieties such as mushrooms, mildew, and mold. Describing all the different kinds would be its own blog entry.
Fungus is all around us all the time, waiting to eat our food, waiting to grow on our toes. It really is the annoying yet lovable roommate that you can't get rid of. And as anyone who has had a truly annoying roommate knows, you just have to be understanding and eat your food quickly.