Hooks in writing are simply a means to grab the reader's attention and pull them into your story. Sounds simple, right? It is and it isn't.
Just like in fishing, prose hooks come in different styles.
There are beginning hooks. You find these at the beginning of a book. They can be a paragraph, a statement, a comparison or a question. In a single line or four, these hooks set the tone for the entire book. That's right entire book. A book with a funny hook must be a comedy. If your first line is there was a body in my bed, you better be writing a mystery or erotica.
Classic hook: It was the best of times; it was the worst of times. A Tale of Two Cities
There are end of chapter hooks, aka Cliffhangers. These keep the reader turning the pages and usually end with the stakes for the hero or heroine being raised. These too must set the tone of the book. If you're going for adventure, you might end with a choice or a dilemma a la the Lady and the Tiger. No matter which the character chooses, the outcome is always bad, but just how bad it is will keep the reader engaged long after they should have shut the book.
Hook from Redaction: the Meltdown: Please, God. Don't force him to ink another rose onto his arm. He was already fully sleeved.
And lastly, there are hooks for the book. These have to be short and sweet, specific yet generic and they also have to reflect the tone of the book. For the greatest impact, chose words that elicit a visceral response. Here's a list http://socialmediastrategy.org.uk/blog/strategic-marketing/the-top-emotional-response-words-marketing-firms-have-an-inside-secret/ These are used in selling a manuscript via synopsis and also on the back cover of a book.
Book Hook from The Stand: ...apocalyptic vision of a world blasted by plague and tangled in an elemental struggled between good and evil...
Elegantly simple and unbelievably hard to write. And to make matters worse, no one hook is going to work for everyone but if you research your market, you're a step ahead of the game.