Grains of rye grass that have been ground into fine flour that has a slightly sweet-sour flavor. Low in gluten, it does not contain enough protein to develop the gluten necessary for the bread to rise sufficiently without combining it with wheat flour. Rye flour produces a denser and darker loaf of bread. The darkness and flavor of the loaf will depend on the darkness of the rye flour and the amount of rye flour used. Just a small amount of rye flour combined with wheat flour will add a distinctive flavor to the loaf.
The flavor you think of when Rye bread is mentioned comes from the caraway seed.