What counts as an attack is a huge thing that many people get wrong due to going with an intuitive understanding of "attack" as a hostile action, whereas 5e explicitly uses "attack" as jargon for "thing that involves an attack roll, or something else that specifically says it is an 'attack'".
That is rather important because of how different things can key off of attacks, but not effects that require saving throws, for example.
Could throw in the difference between an attack and the Attack action also.