I have a synopsis. I’m working on my cover letter and I realise I need a hook. Only problem is some places advocate a hook should be no longer than a couple of lines. I was reading Jilly,Rabbit and these hooks appear more of a synopsis.

The only difference is they have more of a selling edge. Can anyone clear up this confusion? What is the difference between a hook and a synopsis? Do you need both? Is it the same as the blurb on the back cover?