You start off with fresh bellies (when sliced it'scalled sidepork.)
You soak it in a brine of salt, sugars and curing agent (Tenderquick or DQ cure, cure #1, etc. - sodium nitrite) for a length of time depending on the concentration of nitrite in your cure. A mild brine you can soak for 3 or 4 weeks, a full -strengh brine for a week. You want to make sure you get it pickled all the way through or you'll end up with raw pork in the center. Once cured, this is now salt pork. Great in beans and soups!
Last, you smoke it in the smokehouse. You can cold smoke or hot smoke; you don't want to get the temp too high or you'll melt out the fat too much. Once fully smoked, it is now bacon. However, it's not cooked all the way through; you have to cook it before consuming it (sliced and fried, baked, or broiled).
Those are the three stages of pork bellies.