Nails and screws are totally different. The outlook of their design might look the same, but they are used for different reasons. A nail does not have a thread, for that reason it is hammered into a place. Nails are usually used to hang portraits. Once you would have hammered a nail, you would find it quite difficult to get rid of it.
A screw has two parts to it, a thread and the head. A wrench or a screwdriver is used to join two things via a screw. You can easily unscrew things as they have a head designed for such purposes. To tighten things up the screw is turned in a clockwise direction, mostly. There are four different types of screw heads; flat head, round head, oval head and fillister head. There are different types of the shapes of threads as well. Their shapes define that the screw is suitable for using on which surface.