Background
Natural salt licks have been available for as long as we know and have always provided a valuable source of minerals and trace elements to wildlife, especially herbivores. Since the majority of plants do not provide sufficient amounts of sodium and may lack adequate chloride, salt licks are a necessary part of most free-range herbivore diets. In addition to sodium and potassium salts, vital minerals such as calcium, phosphorus, copper, cobalt, zinc, iodine, iron, manganese and selenium are found in salt licks. In essence the salt lick is a big Flintstones vitamin spread out over the forest floor, and you know that you have to take your vitamins to grow up big and strong! Large numbers of fossils have been found at salt mine sites around the US, as these huge salt licks were a natural meeting place for herbivores and fertile hunting ground for carnivores.
Uses Today
Today, the term salt lick usually refers to a pressed block of salt which is placed out for the use of farm animals. These blocks come in mineralized (trace elements added) and non-mineralized (pure salt) forms. The choice of which block to use depends on the other types of feeds and supplements the animals are provided, but generally foragers need a mineralized block while those animals who are given a significant grain/oat feed supplement do not (as feeds are generally already infused with the same trace elements).
Large Herbivores (Horses, Cows, Deer, etc...)
Salt licks are an especially important part of horse and cow diets during the summer months, when they are subsisting mainly on grass and lose large amounts of body salt through perspiration. The animals will usually take only the amount of salt that they need, although it's necessary to keep a sharp eye on them. Horses and cows that are low on salt will gnaw at wood fences and sometimes strip bark from trees to try and get their necessary minerals. Some people think that their horse is licking their hands out of affection, like a big puppy, but the more likely explanation is that they're craving the salt from your perspiration.
Salt licks are sometimes placed out for wild deer, as salt-deficient deer will strip all the reachable bark from trees, killing them (who says humans are the only bad conservastionists?). Of course, they are also sometimes placed out by hunters as bait. Depending on state, this may or may not be considered "baiting" and carry a hefty fine, although baiting usually refers to placing corn or grain out. Your local wildlife officer will know your state's laws regarding the use of salt licks.
Lots of pet stores sell miniature salt licks for small herbivores, but for 99% of "pets", they are completely unnecessary. Any small pets feeding mainly on pellets are already getting their salt and minerals directly from the feed, and don't need an additional salt lick. So unless you're feeding your pet completely natural, organic food, tell the salesperson down at PetsMart exactly where to stick that block.
Anecdotes
If you've read my corncob pipe w/u, you may have guessed (correctly) that I was raised in the country. The deep country. Banjo pluckin' and cousin fuckin' country. My parents raised horses, and we kept salt licks out for them all summer. My sister and I were salt lick crazy.
That's right...we licked the same salt that our horses licked. How much closer to nature can you get? Dad built a combination hayrack, grain trough and salt lick holder in our back pasture, and it immediately became our combination jungle gym and fort. Being curious kids and liking salty things, we naturally tried a lick or two and were hooked. I don't know if we were salt-deprived or not, but we certainly went at that block like crack fiends. Our parents yelled and lectured us about it, but they couldn't keep us away. We'd play fort and lick salt till our tiny mouths were puckered and raw, then be unable to eat anything hot for dinner without crying. Once I took Dad's hammer and chisel out and broke off large chunks that we could carry around in our pockets or mouth. I got a blistering whipping for that one, since I left the tools out there, where the horses chewed the handles up (horses will chew anything they think might just possibly be edible).
This lasted all of one summer. I guess I was about 6 and my sister 4. I can clearly remember anticipating summer and salt licks the next year, but when it came, I was grossed out by the thought of actually licking anything after a horse had had its tongue on it. My sister, to this day, still loves salt. I categorically hate it, and have trouble eating very salty dishes. Neither of us ever got sick because of our dirty little habit—in fact we were almost super-humanly healthy throughout our young lives, considering the number of germs we had to have come in contact with—and I'm a firm believer to this day that the more germs you're exposed to the healthier you'll be.