Does it really matter if I use kosher, table or sea salt?

There are differences in the way kosher, sea and table salt taste—and in the way they measure, too. Kosher salt is courser than table and fine sea salt, so you actually get less "salt" per teaspoon, because it won't pack as densely in the measure. This matters mostly when baking, where the chemical interaction of salt and other ingredients can affect leavening and other texture issues. But for non-baking (savory) recipes, the salts are all interchangeable—after all, you can always add more at the end.

Table salt and fine sea salt measure the same, but table salt has added iodine (it was added in the 1920's to help counter an epidemic of thyroid diseases)—which alters its flavor. You may not notice the difference in flavor unless you taste them side-by-side.

For information on different types of salt and how to use them, see 7 Ways with Salt.