These terms in some instances seem to be conflated, or have very similar definitions. For instance, here's a quote from ScrewFix's website on their 'self-tapping and driving screws page' (emphasis mine):
Self-tapping screws are designed to tap their own hole in the desired material, meaning that no pilot holes needs to be drilled before hand.
They make no mention of self-drilling, leading me to believe the two terms are synonymous. On the other hand, ToolStation assures me they are not the same:
Tapping is when screws create threads in the material they are driven into. Whereas regular screws are typically driven into specially prepared drill holes, a self-tapping screw makes its own threads as it enters. However, they still require a pre-drilled pilot hole...
Self-Tapping Screws vs Self Drilling Screws... The main difference is that self-drilling screws do not need a pilot hole
These are both large UK-based trade stores, so I don't think this just a difference in UK/US terminology.
So, what is the difference between self-tapping and self-drilling? And what does "tapping their own threads" mean?