Can someone help me understand how shopping carts work?

I've never used one before because I've never had a product to sell. But I'm curious about how they work. Like when someone orders a product from your site, do you get an email or do you see it in your account on the website? Is it something you install into your control panel or something you put into your source code (not sure if I'm using the right terms)? If someone could just give me a link to a website that has the info that I need, I would really appreciate it. I tried asking on other websites but the people always get nasty and sarcastic as if I already know this stuff. I don't know anything about ecommerce which is why I'm asking for help.