In a drum brake, the friction part is not a flat pad but a curved piece called a brake shoe. It does the same job as a disc brake pad, in a different shape. Here is what a brake shoe is.
What a brake shoe is
A brake shoe is a curved metal frame with friction material bonded or riveted to its outer face. Two shoes sit inside each drum, and when the wheel cylinder pushes them outward, that friction lining presses against the drum to slow the wheel - see how drum brakes work.
Shoe vs pad
A shoe curves to match the inside of the drum and pushes outward; a disc brake pad is flat and squeezes inward on a rotor - see shoes vs pads. The friction material is similar, but the shape and mounting are completely different.
How shoes wear
Like pads, shoes wear down over time as the lining thins. Because rear drums do less of the braking than the front - see brake wear patterns - shoes often last a long time, but they can wear unevenly or get contaminated by a leaking wheel cylinder.
When to replace
Replace shoes when the lining is thin, cracked, or soaked with fluid, and always replace them as an axle set. Service the drum and hardware at the same time. Keep the front discs fresh with quality pads and hardware.