Describe the meaning "being in love" verses lusting for someone.?

NOTE: this is not something i am pondering over, just out of mear curiousity what others views on being in love and being attracted to someone and caring for them deeply.. is there a differance? i have a friend that say everytime she ends up with someone knew she says "im soo inlove with this person" always saying this one is the real deal to then find out it wasnt the one for her. so there ya go.. have at it and in your own words explain how you view being in love with someone and knowing w/o a shadow of a doubt that this is the "ONE"