You can find it for sale online in up to 5 lb bags, there's a huge variety of flavors and types and prices, but basically it's salt that has been smoked til it almost turns black.
I tend to like the really over-done ones, with names like "Bonfire", that just smell like the house burned down around them.
Too lazy (as I often am) to stoke a smoker all night? Cooking a pork shoulder in the oven? (as I often do...) just coat the outside in smoked salt, and then finish with a bit more after pulling... it really hides the fact that you're a lazy city-dwelling Yankee.:-)