Love / In Love

We all know that there's a fundamental difference between loving someone and being in love with that person? But what's that difference, exactly? People say that it's really hard to explain, but when you're in love, you'll know. So how do you know? What is it that tells you you're in love? A certain vision with that person?

Remember back in high school (or earlier or later), when you had a crush on someone or went out with him/her? You thought you knew what love was. Years later, you realized you didn't. So if you know now, how do you know you know? How do you know you won't find later that you still had no clue today?

So ... what's being in love, and how do you know?