It's that wonderful all-involving adoration and respect you feel for someone who you know is right for you. It makes you lose your appetite and makes butterflies fly in your stomach the moment the phone rings and you hear their voice. Their presence leaves you breathless. There's an aching in your heart and a longing in the pit of your stomach when you're not together.
Then your bank account slowly starts to empty...
That cannot be answered as, well... I believe it is a kind of X-factor that if we new what it was, we would abuse it (I'm cutting a long explanation very short here). It's as if it is beyond our comprehension to understand it, just like my cat looks at me like I'm crazy when I'm getting dressed in the morning, (purple and red are beyond my cat's comprehension).
Love is a more respectable term for the feeling where you think that you can spend your entire life with someone without getting bored of them and (usually) that you can continue to feel lust for them for the rest of your life.