White and black sesame seeds are a common garnish in Chinese cooking, where they are used for their color, taste, and crunch. The white seeds should be hulled when you buy them. For use as a garnish without further cooking, they must be toasted briefly in a dry skillet over low heat, stirred until golden. The black seeds may be toasted if you like—it brings their aromatic oil to the surface—but it is not necessary.
White and black sesame seeds are available in Oriental groceries, in small plastic pouches weighing from 2 to 4 ounces. They are a far better buy than the tiny bottles of sesame seeds one gets on a spice shelf in a supermarket, and are usually fresher. If you cannot buy sesame seeds in an Oriental market, purchase them in bulk at a whole foods store where you can first sniff the seeds for freshness. Buy the whitest (actually pale, pale golden) white sesame seeds you can find, and be watchful for packages that contain chaff or a lot of discolored seeds. Black sesame seeds should also look clean in the package; they are often sold as “black goma” under a Japanese label.
I store sesame seeds in airtight containers in the refrigerator or freezer, which is a good policy for any nut or seed with an oil content that is likely to go rancid.
(Parenthetically, I have searched without result to find a distinction between white sesame seeds and black sesame seeds that would give a clue as to how or if they are botanically different. The black seeds are black through and through, which sheds no light on the subject.)