In cooking, salt is hands-down the most often used additive, bar none. But there are actually a multitude of salts available, especially in gourmet cooking, and knowing the difference will help you sound interesting and knowledgeable and pretentious, so let's jump right in.
Salt for cooking generally comes from one of two sources: salt mines or the sea. Really, a salt mine is generally just a dried up sea bed, so I guess it all eventually comes from the same place, but if you were to read a salt industry magazine (and no doubt such things exist) they would tell you that the sources are different, so I will perpetuate the story. These days most salt comes from salt mines, simply because it's cheaper than evaporating or filtering large quantities of salt water, which is your other option.
Normal, everyday table salt, like almost all other salts used in cooking, is basically Sodium Chloride, NaCl, but is almost always iodized. This means iodine has been added, to save you from that all-too-common fate of hypothyroidism (aka a goiter,) something I know you're all very worried about. It also generally has additives to make it pour nicely, and not clump up. Though it is no doubt the salt in 99% of the world's salt shakers, it's actually not what professional chefs (and wannabe professional chefs) use.
People who get really into cooking, tend to gravitate towards either sea salt or kosher salt. Sea salt is just salt that comes from sea water, instead of from the aforementioned salt mines. It tends to be chunkier than iodized salt, and often is used with a salt grinder. Sea salt seems to be popular for some because it is seen as more natural in some way -- don't ask me how artificially evaporating sea water is more natural than picking up salt deposits off a dry sea bed though, because I don't know.
The other salt, kosher salt, is by far the most common among professional cooks. The fact that it's kosher isn't really what's important; that just means, in this case, that it was prepared without additives and presumably according to some religious guidelines. In practice though, kosher salt has a slightly coarser grain, which is easier to pinch and therefore sprinkle to taste, as so many recipes call for. When Emeril Lagasse picks up some salt and goes Ba-BAM! you can bet he's using kosher salt. Kosher salt also has a milder salt flavour than (iodized) table salt, allowing you a little more control over the taste of your dishes.
The uses of salt are far too myriad to go into, but it's a safe bet that you can add it to just about every recipe, and indeed, you'll find it's already included in most of them. One handy cooking property it has, aside from taste, is that it helps to prevent pasta noodles from sticking, and therefore is almost universally added to water used for cooking pasta.
I got a great tip a while ago from a TV chef named Bobby Flay about the various salts: a taste test. He suggested getting a crisp vegetable like celery or cucumber, and dipping a piece in each of the salts, to see how they differed in taste and texture. If you're just getting into cooking, this is a great idea, and has the side benefit that, whatever you decide, you will end up with some of each salt, whatever's left after the tasting.