If you don't know what type of operating system you're using, it is probably Windows. Microsoft sells their system directly, but most people have it because it comes with their computer. The price is bundled into the cost of the computer and because stores can buy a whole bunch of licenses at a time, it is usually about worth $30 to them where you'd pay between $120 and $250 if you bought it yourself. It is important to note that it isn't free even if you didn't intentionally buy it. Some people buy computers then return the unused Windows license for a refund, but most people find it to be more trouble than they're interested in.
Beyond Windows for the home computer, laptop and phone, there are also server versions of Windows for businesses. As that is a constant bane and funding of my life, you'll probably find articles about those here.