Beautiful, beautiful song. Written by Kannadasan, composed by Ilaiyaraja and sung by K J Yesudas. One of the greatest songs of all time. To me, it's one of the finest examples of pathos expressed in a song. Yesudas' voice is perfectly suited to this sort of mood.
However, what I didn't realize until a friend of mine named Rajesh told me, was that Ilaiyaraja hahd used a little trick t enhance the effect of his voice. If you listen very carefully to the song, you will notice a single violin playing the tune that Yesudas sings. It plays only when he's singing, not otherwise, so it's difficult to catch. But once you do, it's hard to miss afterward. It's a really small thing, almost trivial, but it makes a difference.