The vast majority of recipes call for unsalted butter, but many people keep salted butter in their homes for everyday use, like spreading on toast. The difference is, unsurprisingly, the amount of salt in the butter. Unsalted has none while the amount of salt in salted varies.
Unsalted butter, also known as sweet cream butter, will have the freshest, most natural butter taste. Salt is a preservative and was originally added to butter to keep it fresh longer when refrigeration was not readily available. Unfortunately, salt can be used to hide off-flavors, disguising low-quality butters and butters that are not particularly fresh. If you know that your butter is a high quality, feel free to use salted or unsalted as you choose. At the rates that butter is produced and sold, you do not really need to worry about finding spoiled butter in the market, but you will want to replace your butter if it sits in your fridge for a long time. Unsalted butter stays fresh for 2-3 months, while the shelf life for salted is about 5 months. You can wrap your butter in foil and store it in the freezer, as well, which will keep it fresh much longer.
While the amount of salt in your butter, should you choose to use salted, will not affect the performance of the butter in applications such as baking cookies or frying an egg, it will affect the taste of the final dish. If you are sensitive to salt, you will probably want to cut back on the salt called for in the recipe, since salted butters can contain from approximately 1/4 to 3/4 teaspoon per half cup. If you don't feel comfortable altering the ingredients in your recipe, than keep unsalted butter in your fridge for cooking and save the salted for toast.
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