Brake fluid is a type of hydraulic fluid that has a high boiling point to withstand the heat from drum brakes and disk brake calipers. When you press the brake pedal in your car, it compresses the brake fluid causing it to activate your brakes to stop your car. Brake fluid is hydroscopic, which means that it absorbs moisture from the air rapidly. For this reason, you never want to leave the cover off of your brake fluid reservoir for long periods of time, as the fluid will absorb moisture.
When hydroscopic fluids absorb moisture, the boiling point of the fluid becomes much lower and it takes away from the fluid's ability to compress. Effectively, moisture in your brake fluid limits the ability for the fluid to make your brakes slow down your car. Moisture in brake fluid rusts out the brake lines, which are steel, as well as the brake components themselves (the calipers, drums, etc.)
Since brake fluid absorbs moisture naturally over time, we discourage drivers from keeping half-empty bottles of brake fluid on the shelf for future use. Moisture in your brake fluid decreases your car's braking performance and causes corrosion to your braking components and we strongly recommend periodically flushing your brake fluid. Think of it as a cheap insurance policy on your braking system.
Brake fluid will eat through the paint on your car, if you spill it, be sure to wipe it up immediately. Don't let grease or oil get in your brake fluid reservoir - it will cause significant (and expensive) damage to your brakes. If you leave a brake fluid bottle with the cap off for 15 minutes or more, it is likely ruined because of its hydroscopic nature.