Can anyone explain to me how hot water heat works?

I live in a apt. building built in the 60's. I am top right floor, boilers are bottom left. I have long low wall "registers" in each room. Are there actual pipes that run the hot water through these? They are metal, and bang when they expand from the heat, but since I don't have to pay for heat, I can live with that. How does the heat get out into my rooms - there is no blower. Also, I have a thermostat - could it possibly even be hooked up to anything, or is it just there to make me feel better? It is so hot in my apt sometimes that when the thermostat is on 60 I have to open my windows, and at other times my kitchen floor is so cold my eyes pop out when I step on it. Can anyone explain the mechanics of this to me?