What the heck is rennet anyway?

This is an excellent question, and in the interest of blissful ignorance I've chosen to believe it's a magic cheese-making pill that comes packed in foil in a little box.

After a couple of great batches of ricotta, I wanted to make some other cheeses but all the recipes I could find required (at the very least) something called a rennet tablet. So the next time we were at our local Giant I asked about it; You can make your own cheese? Ask Trader Joe's, they said.
So the next time we were at our local Trader Joe's we asked again. They said, Nobody bought it. Try Whole Foods.
So at this point I called Whole Foods. The woman I eventually reached immediately knew what I wanted and also had this answer: No, we certainly don't carry anything like that. Try Wegman's.
And, can you believe it, the guy at Wegman's said, Why yes. In Aisle 15B.
By this time I had read several rennet cheese recipes and was beginning to wonder at the wisdom of storing cheese for 6 months (which seemed to be the usual resting time for hard cheese) in my lovely yet life-prone kitchen.
Happily, as we were shopping on this cool July 4th morning, we were completely detoured from our way home by a parade in Towson. So we ended up on the highway and J said, I think I know of a Wegman's around here.
Long story slightly longer, I actually remembered the aisle number three weeks after calling and found the VERY LAST box of rennet they had, hidden with the gelatins. And only an investment of $1.67 for 8 tablets, cheaper than the milk!
And so began this newest saga.
I now have a fresh gallon of 2% milk, some live & active yogurt, and brand-new cheesecloth. Off to rennet cheese #2!