What is the difference between bolts and screws?

A fastener or fastening is a hardware restraint that mechanically joins or holds two or more objects together.
In general, fasteners are used to create non-permanent joints.

Bolts and screws are typical fasteners.

What is a bolt?

Bolt is an externally threaded fastener for the assembly of two unthreaded components,  which is normally tightened with a nut.
There are dozens of different types of bolts including hex bolts, flange head bolts, carriage bolts, round head bolts, countersink head bolts, elevator bolts, J bolts, lag bolts, rock bolts, shoulder bolts and U bolts.

What is a screw?

Screw is usually used to fasten materials by digging in and wedging into a material, while the thread cuts grooves in the fastened material that may help pull fastened materials together and prevent pull-out.
There are many screws type including chipboard screws, drywall screws, wood screws, machine screws, self tapping screws, self drilling screws, security head screws and sheet metal screws, etc.

Some can't tell the difference of a bolt and screw, and they say bolts are screws. But, not all screws are bolts.

So what is the difference between bolts and screws?
 
1. Bolts are always used with the nuts, while screws usually worksalone, or used with tapped holes.

2. Bolts doesn't have tapered shank, while screws have tapered shank.

3. Bolts are with spiral thread, while screws are with helical thread.

4. Bolts with nuts are more strong and reliable than screws.

5. The materials to be attached by a bolt should have a hole, while in most cases, screws can be directly insert into the materails to be jointed.

6. Bolts are available in big sizes, while screws with relatively small and medium sizes.

Given below is an image of bolt and screw, as we can see clearly the differences: