At its most basic, peanut butter is simply ground peanuts and salt, which is all you'll find in many natural peanut butters. Super-smooth emulsified peanut butters, which are what's spread on most lunch-box peanut butter sandwiches, contain sweeteners, oils, and ingredients that keep the peanut oil and nut mass from separating.
The two kinds differ enormously in texture and flavor, with the natural peanut butters having a more coarse texture. Using one in place of another in a peanut butter cookie, for example, will give you different results. Both natural and emulsified peanut butters also come in a chunky style, which contain peanut pieces. In baking, peanut butter is used in cookies, candies, tarts, icings, and fillings. On the savory side, it's the featured ingredient in peanut sauce for satay and often in sesame noodles.
Another nut butter (almond, hazelnut, etc.) can often substitute for peanut butter, though the flavor profile will be different.
Choose peanut butters with a true peanut flavor. There are many varities on the market, so try a few to find your favorite.
Natural peanut butters often need to be stirred because the oil separates out.
Natural peanut butter must be refrigerated and will keep about six months. Emulsified peanut butters will keep for six months at room temperature.