The oxford dictionary defines 'unrequited' as (of a feeling, especially love) not returned.
It means, being in love with someone so deeply, but not getting the same love back. Either the person has no idea whatsoever of your love to that person, or that person doesn't reciprocate the feelings that you have.
I've never really had an unrequited love experience. I mean I only fell in love once and it ended, but that's a different story altogether. What I'm trying to say is, I've never fallen in love otherwise, but I did have my own share of crush unrequited though. It may not be the same as unrequited love, but it fits the category, because when you love someone, you do it with all your heart. On the other hand, a crush is just a form of infatuation, although it may sometimes be mistaken as love - but it isn't. It is just that - a crush.

The crush I'm having right now is definitely only on my side as the guy is obviously attracted to another girl. I see them talking, laughing and flirting almost everyday. It's hard for me to watch, but I just swallow the hurt, and smile not to show my true feelings.
It has happened many times, but it had never prolonged. Like I said, crushes are merely infatuation for someone's looks or appearance. We crush on someone without getting to know them better. If we still like them after knowing them, only then can those feelings turn into love.

You wait and wait for them to respond to what you feel - but in vain. How long do you have to hold on, waiting for that love which you are not sure is even yours?
No one wants to be in that situation, but sadly it always finds us. If you are feeling so, never give up. Never stop fighting, be strong as only the strong prevails.
And if you feel like all hope is lost, look around you and who knows, you might find something else that will make all the pain go away. If you open your eyes and turn away from the misery, there will be rainbow. Though it may not be what you've been waiting for, but you will know that it is what you never realized you've been looking for.