The brake fluid reservoir holds the brake fluid that isn't in the lines and other components of the braking system. The extra brake fluid is there to make up for the extra fluid used in the expanding components (caliper pistons in disk brakes, wheel cylinders in drum brakes). It is typically located above the brake master cylinder, the point where the fluid is drawn in when needed. The cap on the reservoir usually states the type of brake fluid recommended. Typically, the reservoir is only held in place by O-rings and one or two cotter pins.
DOT 3 Versus DOT 4 Brake Fluid
Your brake fluid reservoir cap will likely call for DOT 3 or DOT 4 brake fluid.