When you make love with someone, at last, you move your heart and from your deep heart to shot, " I love you. I really love you." and you almost wanna cry from your heart and you wanna hug this person hard. Then, you will feel of happiness. We call "love".

Desire is just a behavior for short time, then, there is nothing. You just wanna look for excitement, no love, just play. Then, you will feel of "emptimess". We call "desire".

Love is from the heart to the heart,desire is just for the body  (I think)

When in a relationship ...

It's your happiness that matter then it is Desire

and when it's the hapiness of other person that matters it is love

and when both care for the hapiness of each other then it's True Love :)