Yes they are both externally threaded fasteners, but how can you tell the difference?
Bolt- torque the nut
Bolt-externally threaded fastener that is
- designed for insertion through holes in assembled parts
- normally intended to be tightened (or released!) by torquing a nut
Screw- Torque is applied through the head...
Screw-externally threaded fastener that is capable of
- being inserted into holes in assembled parts;
- mating with a preformed internal thread, or forming its own thread;
- being tightened or released by torquing the head.
A bolt is designed for assembly with a nut.
Nuts are required for bolts, optional for screws.
A screw has features in its design which makes it capable of being used in a tapped or preformed hole in the work. Because of basic design, it is possible to use certain types of screws in combination with a nut. Any externally threaded fastener which has the majority of the design characteristics which assist its proper use in a tapped or other preformed hole is a screw, regardless of how it is used in its service application.
Source: Distinguishing Bolts from Screws, US Department of Homeland Security, March 2006.
Speaking of Precision Blog 
So the real questions is…
Why is the Department of Homeland Security deciding the difference between a bolt and a screw?
(Insert joke about nuts here.)
Great question Stick.
Customs needs to assign duty, tariffs and or apply quotas when goods are imported, and the Harmonized Classification System needs to distinguish between screw and bolts for purposes of collecting duties, or penalizing goods that have been determined to be dumped. So the
CustomsHomeland Security Folks need to “know their nuts and bolts” and screws… or at least be able to tell them apart…