Oval heads are countersunk (cone shaped under the head) with a rounded top. Phillips drive is x-shaped for installation with a phillips screw driver. Sheet metal screws have sharp threads that cut into a material such as sheet metal, plastic or wood. They also make excellent fasteners for attaching metal hardware to wood because the fully threaded shank provides good retention in wood.
SHEET METAL SCREWS HAVE SHARP THREADS THAT CUT INTO MATERIALS SUCH AS:
And of course sheet metal
Sheet metal screws are also the go to fasteners for use with plastic wall anchors in drywall. Tapping screws are considered externally threaded fasteners that "tap" their own mating threads in metallic and non-metallic (such as plastic) materials. Tapping screws, especially Type A, are referred to as "sheet metal screws" because one of their first applications was in ventilation ducts made of sheet metal. They also make excellent fasteners for attaching metal hardware to wood because the fully threaded shank provides good retention in wood. The design is often criticized for its tendency to cam out at lower torque levels than other "cross head" designs. There has long been a popular belief that this was actually a deliberate feature of the design, for the purpose of assembling aluminum aircraft without over tightening the fasteners.