Flap Disc

Flap discs for angle grinders - overlapping abrasive flaps on a backing plate that grind and finish in a single operation.

Why they often cost less per job than a grinding wheel

A grinding wheel removes metal and leaves a coarse surface you then have to clean up with something else. A flap disc removes material and leaves a finish, which takes a step out of the job. The disc costs more; the job costs less.

They also run cooler and with less vibration, and because the abrasive wears away to expose fresh grit rather than glazing, the cut rate stays consistent through the disc's life.

Grain by material

Zirconia - the general-purpose choice for carbon steel. Tough, good life, sensible cost.
Ceramic - self-sharpening grain that cuts cooler and lasts several times longer on stainless and hard alloys. Heat is the enemy on stainless, and this is the grain that manages it.
Aluminum-specific - aluminum loads ordinary abrasives almost immediately, clogging the disc dead. Discs made for aluminum use a grain and coating that shed the swarf instead of packing with it. Using a steel disc on aluminum wastes the disc in minutes.

Angle of attack

Flap discs are designed to be run at a shallow angle, roughly 10 to 15 degrees. Run flatter and you glaze the face; run steeper and you wear the edge off the flaps and lose most of the disc.