What is the difference between a bolt and a screw?
More than you might think.
Here are different explanations, all valid in their own way, according to different fastener manufacturers and engineering organizations.
One definition is that a bolt is installed by tightening a nut onto its threads, while a screw either screws into existing threads or creates its own threads. So according to this, a cylinder head bolt is a “screw,” and a pan head screw that uses a nut to fasten together an instrument panel is a “bolt.” (Hmmmm. That just doesn’t sound right.)
Another explanation is that a screw has threads from its tip all the way to the underside of its head, while a bolt has at least some portion of its shank unthreaded. Which means a three-quarter-inch by six-inch fully-threaded hex head “thing” should be called a “screw.” (Yeah, that doesn’t sound right either.)
A third opinion is that a bolt is rotated into place by a wrench or tool twisting the outside of its head. A screw is rotated into place by a tool that engages a drive point in the head, ie, a slot for a flat-blade screwdriver, or a hex socket for an Allen wrench. (Again, hmmmm.)
Personally, I’m going to stick with my long-standing rule of thumb that anything with a hex head is a bolt, and anything with a Phillips-head top is a pain in the butt.