I intuit that hooks are things that hook. (I'm quite astute in this way.) They can be good prose, relatable characters, rich settings.
Books must start with a 'hook.' This is a current truism. The hook is (often) seen as establishing the tone of the book, placing the reader into the setting, into the mindset of a main character. The (implying singular) hook should be the first page of the story, perhaps the first 300 words. Or the first sentence.
My cherished beta readers have indicated that they do not know what it is that my characters want, and it finally dawned on me that this is a stumbling block for them (and myself) - if they knew what my characters wanted, they would be more 'hooked' into the characters.
With this dawning realization I began to wonder, what other elements can be conceived of as a hook? Put otherwise, it had not occurred to me to see character desire, specifically, as a hook. It had not occurred to me to take a broad view of what a 'hook' could be. (It had not occurred to me to hook my readers in as many ways as possible! I saw the readers merely as people to be entertained - not quarry to be poached and reeled in, and now I see that they may ... wish to be reeled in and I'm happy to re-envision my efforts through the lenses of hooks!)
Through the valuable contributions of esteemed contributors on this site, I understand that style, setting, tension, story, and so on are important to writing in a compelling fashion. These can all be considered 'hooks' but are not typically seen or defined as such. I am hoping to gain insight into what does and does not constitute a hook, the more specific the better. I'm not looking to tread over old ground, although I expect that may be necessary. I'm curious as well, what does not constitute a hook. Perhaps there is a reason that setting is not? seen as a hook although it is seen as a necessary component of effective storytelling.