Zack and Cloud are both characters froms Final Fantasy VII, duh. This fanlisting is dedicated to their relationship in all its aspects, though I won't lie, for me it definitely leans towards the romantic. There is such wonderful depth to the sacrific that Zack makes for Cloud that speaks of more than just friendly feelings or comraderie. The lengths with which Zack was willing to go show something even deeper and more precious.
As for Cloud, I think the final goodbye in Advent Children shows just how important Zack was to him. It was Zack's smile and approval and forgiveness, even though none was ever necessary since it was quite the conscious choice in Zack's part to do what he did, that Cloud was able to let his guilt and pain go.