Being "in love" that beautiful, beautiful first phase of loving someone you are physically attracted to. It's all those sexual feelings intertwined with the new-found love. It's that phase when your significant other is the first thing you think of when you wake, and the last thing on your mind as you drift off. It's being able to think of very little...and those really awesome sexual feelings....the feeling of not being able to live without the other.... the intense longing, both physically and emotionally. It's all-consuming. Enjoy it while it lasts, because, alas, it does not. After "in love" will be plain love, but that's a beautiful thing in itself. :)
Being " in love" refers more to the romantic feelings of the beginning stages of love. When those butterflies calm down, when you become more use to the other, you get to the dedicated part of the relationship. That commitment to stay together is because of love. Love is a commitment.