Fancying someone is what happens when you meet. It's largely superficial - you might like their looks, or you might like what you see of their personality. But you don't really know the person.
Being in love is what happens when you know the person. Their looks no longer matter. Their personality certainly matters - but not the personality they show when you they first meet people, the one you fancied. By now you know their real personality, the one that comes out when they come home after a long day and don't want to socialise, but being with you isn't socialising, it's just part of life. When you know this hidden person, and still "fancy" him/her just as much, you're a fair way towards being in love.
