What is really love?

One day you meet this person that you haven't seen in a while, And I say meet because after all those years people change.... You talk, you laugh, and keep talking, and the time goes by, and suddenly you discover yourself kissing and hugging, and feeling great... and enjoying the time with this stranger that happens to be part of your life now.
It happened to me, that when I wasn't searching for love... when I wasn't searching for that somebody he knocked in my door, I didn't expect that to happen, how was I supposed to know... now I feel this way, I can't do anything about it and I don't have him anymore. My stranger love went far away and there is nothing I can do.
Maybe there is but I just don't want to admit it, or maybe I am just afraid of what could happen if I decide to do something about it.
So after all, what is that feeling called love?, how can yo tell?, How do you know when?, is it reality, is it just in our imagination? What is really love?