Screws and bolts both belong to the family of threaded fasteners. They were some of the early fasteners in use and before the advent of a wide variety of fasteners suited for different applications, the terms screws and bolts were often used interchangeably.
A crude way of differentiating a screw and a bolt is to state that a threaded fastener with a nut and a flat bottom is a bolt and one without is a screw. However, the most suitable way to distinguish a screw and a bolt is comprehending that, in essence, the main difference between a screw and a bolt arises in the way they are used.
Screws and bolts are both externally threaded fasteners. Bolts are often restricted from turning when part of an assembly, and have to be loosened by applying torque on the nut. On the other hand, screws can be placed within pre tapped holes and can penetrate materials. They are often fastened onto the materials by applying a torque onto the screw head.