I'm going to give you a link to a song. It's so old fashioned that you'll probably hate it, but it's the best explanation of what it means to be "in love" that I've ever heard. I can certainly identify with it, but you'll need to give it some thought. http://www.youtube.com/watch?v=8ihRFBMPaic
It varies from what you think. really it means you want to spend the rest of your life with them I guess. Some people, I can't really explain it, feel something, and they can never get "that person" off their mind.
It means that u will always be their for them. U will also help them in any way possible and be LOYAL to them. No cheating. U also can say things and be open about things to him/her. And u should treat them with respect. Their not just a toy; their a real person with real feelings too.