Disc brakes are made up of a brake caliper, two brake pads and a flat metal rotor that spins in tandem with the wheel. When the driver applies the brakes, the caliper squeezes the brake pads onto the rotor. This causes friction that slows the spinning of the wheel.

It's only loss it's high expensive nd benefits are given below

• Less brake fade. 
• Better performance in wet condition.
• Less likely to lock up.

A  drum brake consists of a small round drum that contains pistons and two brake shoes. The drum rotates next to the car’s wheel. When the driver depresses the brake pedal, pressurized brake fluid is sent to the drum brake, and it forces the pistons to push into the brake shoes. This action causes the brake shoes to press against the sides of the drum, creating friction that slows the spinning of the wheel.

benefits it's less expensive .it's loss are given below

1.Less effective at dissipating heat. 

2.Less effective in wet conditions. In wet conditions, water has a tendency to pool inside drum brakes. This negatively impacts braking performance.

Disk brakes work on hydraulic (liquid power) system where in pressure is created on a disk attached to the freewheel. the fluid pushes the brake shoes from both sides of the disk which brings the disk to stop (naturally the wheel to stop). It is just like the way you hold a spinning disk by your hand. It is visible from the outside (obvious). Drum brakes are not visible from outside. they form the center of your flywheel. the basic idea is that if the center of the wheel stops, the wheel obviously stops. This mechanism works mechanically (using wire). The break shoes aren't visible from outside they are in the drum (center of the wheel). when brakes are applied, the string/wire pushes the break shoes against the inner side of the drum bringing the wheel to a halt. Drum breaks and Disk breaks have no difference in their effectiveness. They are both effective. It is just the design which suits the bike/car. Disk brakes look shiny giving complimenting the rims and giving it a elegant look.