There are many uses for hydraulic oil, and hydraulic brake fluid is just one variant. Hydraulic brake fluid is made of refined mineral oil or water depending on the brand you have used. It is basically a special form of hydraulic oil that is used specially for braking. These materials have a high boiling point that makes for a perfect braking fluid. These qualities are needed, as there is a large amount of heat produced during braking. Prolonged braking over long distances would cause the brake fluid to evaporate, but this does not happen.
For more info, see 4 Top Brake Fluid Brands