Washer head screws have a lo-root thread and a washer shaped head to prevent the head from pulling through. Washer head screws are faster and easier than using a flat head screw plus a finishing (cup) washer.
The 1 1/8" length is ideal for attaching two frameless cabinets together, the 2 1/2" for anchoring cabinets to the wall.
Sold in packs of 100, 1000 or bulk box (quantity depends on length).