Like nails, screws are (usually) small pieces of hardware used to hold materials together. One of the main differences is that screws are designed to be able to be removed and replaced easily without damage to the screw itself or the material it connects. Most screws are made from some type of metal, and many are covered in a rustproof coating. Aside from material, screws differ in head shape, head type, threading and purpose.
Machine screws are designed to hold pieces of machinery together. Most machine screws require that the hole already be drilled and threaded, although there are a few types of machine screws capable of tapping their own holes. Like other screws, machine screws are available in a variety of shapes and sizes. Pan head machine screws are machine screws with heads that are flat on top and rounded on the sides. They're similar to oval head machine screws, which also have rounded sides; the difference is that oval head screws have a rounded top, too, as opposed to a flat top, and they have tapered bases. Both are commonly used for metal applications. Pan head machine screws can also be differentiated from flat head screws, which have flat tops, as well. However, flat head screws are tapered on the underside of the head so they can lie flush with the material they're driven into. Meanwhile, round head screws have flat bottoms with rounded heads, much like half a sphere. Round head screws can't sit flush with the material they're driven into; instead, their heads poke out like little bumps.
Pan head machine screws can be designed with a variety of head types, too. A slotted pan head machine screw has a single cut in the head for use with a straight screwdriver. Philips pan head machine screws have a cross cut into the top for use with a Philips screwdriver.