This song was sang by Yesung. It is the OST for a Korean drama, "That Winter, The Wind Blows", of which I did not watch but heard many people talking about it when the drama was aired. Because of military service, Yesung could not perform the song; hence, his two counterparts sing on his behalf until he complete his service.
This song has meaningful lyrics. Go to 03:16 and continue watching ~
When Yesung's (singing) part was being played at 03:43, tears just flow uncontrollably. It's really nostalgia. This is the first time, after so long, that I'm experiencing a sudden surge of emotions despite umpteenth time I watch it.
It's hard to find a song that has the best combination of lyrics and the singer's voice; yet, Yesung does it. :']