Don't know that it's possible to describe what it is, only the actions it causes, and a relationship isn't necessary.
You can love someone you can never be with. You can hate someone you love. Despite such hate, you still want the best for them, and you want them to be happy. You miss them, even though you might hate them. It's more obvious to everyone else than it is to you how much you love them. It can inspire you, or destroy you.
I'm only referring to romantic love by the way, not platonic. That one is far easier to understand. Platonic love is what's felt when someone is someone you trust, has your loyalty, and you care about.
Despite my appearing bitterness, I do know what a positive love is. We were a team, always wanted to be near each other, always wanted to do everything together. The love was passionate, the fights were brutal. All emotions were amplified intensely when it came to her and I. I was the only one who could make her cry, and the only one who could actually make her truly happy. Same applied in reverse.
We still see each other occassionally, and the sparks still fly when we do, like it was shortly before we got into a relationship, and the first few months. The only thing preventing us from still being together is the knowledge of how much we end up hurting each other. It was a back and forth between bliss and hell.
Meh... that's all I have to say. Just a rambling.