A paste made of ground sesame seeds, tahini has a rich, nutty flavor that helps to amplify the flavors of other foods. Tahini gives Middle Eastern dips like baba ghanouj (eggplant dip) and hummus (chickpea dip) their wonderfully rich and creamy qualities, and it's full of B vitamins, calcium, phosphorus, and iron.
Peanut butter or other nut butters can sometimes be used in place of tahini, but the flavor will be different. Asian sesame paste, which has a darker, roastier flavor, can also substitute.
Imported and domestic tahini is sold in cans and jars at natural-foods stores and some supermarkets (look in the international or natural-foods section or next to the peanut butter). A good domestic brand to look for is Joyva, made in Brooklyn, New York.
Most commercial tahini is made without added emulsifiers, so the oil usually separates from the solids, as natural peanut butter tends to do. If you have time, let the container sit upside down on the counter overnight and gravity will remix the tahini for you. Otherwise, put the separated tahini in a food processor and blitz it until smooth.
Tahini keeps almost indefinitely when stored in a cool, dark cupboard. It doesn't require refrigeration (sesame oil, unlike other nut oils, is high in antioxidants), although you can keep it in the fridge if you like (just let it return to room temperature before using).