Yes they are both externally threaded fasteners, but how can you tell the difference?
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-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.
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. Bolt Screw Nut
Many times a print will come in to be quoted with material called out to be “Grade 8,” “Grade 4.8, “, “B7” or something similar.
Where can you find the material, chemistry, process, and mechanical requirements when given these ‘clues?’ Hint number 1: If the dimensions are customary US units, try SAE J429 Mechanical and Material Requirements for Externally Threaded Fasteners. J429 calls out grades 1, 2, 4, 5, 5.1, 5.2, 8, 8.1, 8.2 J429 is available from SAE. J429 link. Hint number 2: If they are metric, try SAE J1199Mechanical and Material Requirements for Metric Externally Threaded Fasteners. J1199 calls out 4.6, 4.8, 5.8, 8.8, 9.8, and 10.9. J1199 is available from SAE here. SAE J1199 link. Hint number 3: Bolts with a “B” prefix like B7, B7M, B8, B8A etc can be found in ASTM A 193 Alloy Steel and Stainless Steel Bolting Materials For High Temperature Service. available from ATSM. A193 link.
One final thought. These are not the only sources of bolt grade designations by number. ISO 898-1 is another document for metric bolting materials.
Photos credit- PMPA Active Member Huron Automatic Screw Company.