Differentiation between bolt and screw
A screw, by definition, is not a bolt. A bolt is designed such that a nut (or other turning device) is required for operation. A bolt is not designed to be turned. What most people refer to as a bolt is in fact a 'cap screw', which is designed to be turned (or screwed). Cap screws may, or may not be used with nuts. The distinction is subtle, but significant in the design of the fastener. If threaded all the way to the back of the head, it becomes a 'machine screw'