Demerara sugar is, like Turbinado (or Sugar In The Raw), a cane sugar which has been less processed than white table sugar. It still contains a proportion of impurities which give it color and a hint of flavor.
When you process cane sugar, you get white sugar and molasses at the end of the process. White sugar is white sugar; molasses is all the impurities they refine out. Clever of them to make you pay for trash, innit?
Note Demerara sugar is not the same as brown sugar, which is white table sugar with molasses added back!
While Demerara and Turbinado sugars are different, in brewing quantities the differences are not pronounced. Thus, if you cannot find Demerara, Turbinado can be substituted with a clear conscience.
Cheers (and thanks for your interest),
Bob