A screw is a small metal post with threads going down around it. On the end of the post is a head with different types of indentations, depending on the screw it is. A screwdriver tool is used to insert and remove the screw from the hole. To insert or tighten a screw rotate it to the right, to loosen a screw rotate it to the left. An easy way to remember this is with the saying "righty tighty lefty loosey".
There are different shapes, sizes, and screws that you are likely to encounter while working on a computer. In the above picture is some examples of different screw heads you are most likely to encounter while working with a computer or computer hardware.
Although most people have the proper screwdriver to work with a computer, there are some cases and hardware that prevent access to an end user with tamper proof screws. If you are looking for specialty screwdriver accessories, search for available solutions.