Many people think that a screw and a bolt are different primarily in their heads. Screws have slots for screwdrivers. Bolts are said to have polygonal heads and are wrenched tight. None of that is true.

In basic mechanics, the difference between a screw and a bolt is solely the means by which it is secured. A screw is threaded into the object it means to attach to. Wood screws are the archetypical screw, because the threads attach directly to the wood. In metal, if the screw attaches into a threaded hole, it's a screw. In engines, the head bolts aren't really head bolts, they're head screws because the block is tapped and the 'head bolts' attach directly to the block

A bolt is held in place by a nut, that is separate from what the bolt is used to secure. Boltholes are not threaded, and the bolt extends through them to be clamped by a nut the other side.

The short rule is, if it has a nut, it's a bolt. If it doesn't, it's a screw. I hope this clears things up, but expect that I won't change common usage.