Sesame seeds have a hard outer shell called hulls or husks that can be removed. Hulled sesame seeds are seeds that have this outer shell removed to rid of bitterness the husks might have. Un-hulled sesame seeds are usually popular in Japanese cuisines, however, both types of seeds can be easily found in grocery stores.
Hulled sesame seeds are completely white and have a milder and nuttier flavor than the unhulled variety. Sesame seeds that still have their husks on usually have a stronger flavor are usually used with other complementary spices. Un-hulled and hulled sesame seeds are usually interchangeable and both types of sesame seeds carry the same nutrients and antioxidants.
However, you should consider adding more unhulled sesame seeds to your dish if you want to achieve the same nutty and earthy flavor as if you were to use unhulled sesame seeds. Otherwise, if you are looking for a simpler subtle flavor or just another layer of texture to your recipe, hulled sesame seeds would be perfect.
Hulled sesame seeds are best added to desserts and bread as it adds a different kind of sweetness and rustic flavor.