You feel like they are your entire life, you can not stop thinking about them even when they are not around they are on the center of your mind. You also know because you would be willing to do things for them or with them that you wouldn't with just anyone.
I would want to understand more about the person, and want to involve the person in my life. I start thinking about a future when we will be together, and she starts becoming more important in your life that you feel lost if she wasn't there anymore.
Oh man it must suck to have feelings for someone and have them unreturned. Never has happened to me. Since I'm so gorgeous, intelligent, and suave I often have multiple women developing feelings for me. I find it hard to develop feelings for inferior American women.