To be in love is to consider the other person your best friend, your 'soul mate'. You've thought about the future with them, having a family with them. You love them no matter what, you help them with whatever they're going through. While you still have eyes and can obviously see other members of the opposite sex, you only really have eyes for them.
To fall out of love... I don't know. I'm not sure you truly can unless the other person has done something wicked to deserve it, i.e, cheating, been physically abusive. Everyone has moments in the relationship where they have doubts. You might doubt your love at those points, but I don't think you can fall out of love with someone as quickly as flicking a switch for example. I was in a relationship where he told me he had fallen out of love with me, but a few months later he said he never had and was just having a freak out because we had been together for 2 years. He begged me to take him back, and even though I'm still in love with him to this day, I just can't mend that hurt. It sounds silly, I know, but there's a window after someone tells you something like that that you might be able to go back, but it was months of no contact, and I had effectively grieved.
Everyone's experiences are going to be different, that is just mine.