I fell down the stairs and now my left shin really hurts. Its elevated with ice and a large bruise is forming. I can walk/limp. What did I do to it?

Bad bone bruise. The shin normally has a thick layer of skin, then periosteum (thick bone cover), then bone. You gave it a hard knock, bled under the skin and got inflamed and lots pain. If u can walk, it is very unlikely u broke anything. So continue ice, elevate, switch to heat after 48 hours, mild analgesic as neede for pain. Oh, and why did u fall? Poor lighting? Don't let it happen again, you hear!
Contusion. Sounds like you have a contusion of your shin with a big hematoma or collection of blood as a result of your injury. Contusions directly over the shinbone are more likely to be severe because there is little room for swelling in this part of the leg. If the pain is out of proportion to the severity of the injury, you should go seek immediate care. There is a serious danger of compartment syndrome.
Contusion. It sounds like a contusion and/or a bruise..Your treatment is appropriate.
Most likely. Most likely you contused it or bruised it. You are doing the right thing by icing it down. If the pain persists or worsens, get checked out to rule out a fracture.
Most likely. Most likely you bruised it or contused it. You are doing the correct thing by icing it. Fortunately it is quite rare to fracture the bone there. But if the pain cannot be controlled you will need an x-ray to rule out a fracture.