It's flour made from milling rye grain—a cereal grain—and comes in different varieties such as light, medium, or dark, depending on how much husk and germ have been removed. This flour is not gluten-free but contains far less gluten than regular or wheat flour and is much easier to digest. I love the taste of rye flour in cakes as it adds an earthy sourness that balances any added sweets. It also adds moisture and therefore results in a denser bake. If you've never baked with rye flour before, I suggest using light rye flour at first, as a darker variation is more intense in flavors but, of course, also more nutritious. u003cbr/u003e