Rock Salt is a sedimentary rock (so it's found on the top layer). You can mine it directly using a pick axe to get 0-3 rocks and a chance for a gem. If you wish to get the block as an item for building/decorative purposes, you must mine each of the six blocks touching the faces of the block you want. This block cannot be used as a stone anvil.
The rocks from mined rock salt can be ground at the quern to produce salt. The main purpose of Salt is to extend the shelf life of raw meats. Combining salt with any type of meat will reduce decay-rate on that meat by half.