to the naked eye, there does not appear to be much difference between a bolt and a screw. They are both threaded fixings and have a head for tightening the fastener but there is more to it than that.
The most obvious way of differentiating between a bolt and a screw is that a bolt is not usually threaded all the way along its shank as it has a plain portion. A screw, however, is threaded fully to the head.
Bolts are designed to be installed with a suitable nut. The hole for a bolt is not tapped as the bolt is pushed through and is fixed and tightened using a nut at the back of the material being fastened. So a bolt will be longer than the width of the material it is being used on, as it needs to protrude through to the other side to screw into the nut. The unthreaded portion of the bolt (which sits inside the material) adds strength, making it more resistant to shear forces, compared to fully threaded screws. Bolts are usually fastened using a spanner or other tool which grips the head whilst the nut is tightened.