In its most simple terms, butter is cream churned until it reaches a semi-solid state. Among its roles in savory cooking, butter makes a flavorful fat for the sauté pan and a luscious thickener for sauces and gravies. It's delicious tossed with vegetables and makes amazing mashed potatoes. Butter plays an even bigger role in baking with its tenderizing properties and irreplaceable flavor.
Some grocery stores now carry higher fat, European-style butters. More fat means less water, which in turn means flakier, more tender, and more flavorful pastries. These butters are a wonderful splurge, but they're not a must-have for good baking. As long as you use fresh, unsalted butter, your recipes will come out well, no matter the brand.
1 American-style stick of butter = 4 oz = 8 Tbs.
Oil can replace butter in the sauté pan, while vegetable shortening will provide some of the same tenderizing powers in baking (pie crust; cookies), but without the same rich flavor.
Most cooks prefer unsalted butter (also called sweet butter) to salted for several reasons. The amount of salt in salted butter varies by brand, which can cause inconsistent results in recipes. Another reason to avoid salted butter is that the salt acts as a preservative, which means that salted butter can be kept longer on market shelves and isn't always so fresh; salt also masks off flavors.
Pay close attention to the temperature your recipe indicates for the butter. Many baking recipes call for softened or "room temperature" butter. Such butter should be pliable but not too soft for best results. To soften cold butter quickly, cut the butter into small cubes and let them sit on the counter in a single layer; they'll reach optimum temperature (65° to 67°F) in about 30 minutes.
To keep butter as fresh as possible, store it tightly wrapped away from light on a back shelf in your refrigerator (the butter compartment on the fridge door is actually not the best place; the temperature fluctuates too much there). You can freeze butter, too. It may seem watery as it thaws, but you'll find little difference in the final baked product.