In On Humanism I laid out the foundation of humanism: the promotion of happiness and the diminishing of "suffering". Both terms require more investigation; since "suffering" is easier, I'll start with that.
Suffering is not pain per se, although one can suffer from pain. However, it's possible that one might not suffer at all: The athlete "feeling the burn" is feeling pain, but enjoying it. There are those who receive sexual gratification from pain; one cannot justly say they are suffering.
Suffering is likewise not disappointment per se. When I lose a game of Go to my sensei, I am disappointed in a sense, but I was also after my pleasure in being beaten well, and learning in the process.
Suffering is bound up strongly, I think, with will: Suffering is, in the main, the forceful contravening of the will. I want to speak my mind, and I want to move about freely; if I were threatened with imprisonment for my views, my will would thereby be contravened and I would suffer. I want my loved ones to be with me perpetually; when they die, I suffer.
Because of this strong bound with will, suffering is strongly subjectively constructed. You cannot tell me what constitutes suffering for me (nor I for you), because of all the things I know, I know my own conscious will the best.
Context and level of abstraction are also important. One can be said to be suffering in one sense and not suffering in another: A person such as Eugene Debs, imprisoned for opposing WW-I, was suffering in the sense that ideally he wanted to be free, but he was acting in accordance with his will to speak out against WW-I, and thus on that level was not suffering. I might choose to live in relative poverty to save money and later become wealthy, and thus am acting in one sense according to my will, but I would prefer to be wealthy without the concomitant poverty.
The differing levels of abstraction come into play especially when discussing the Problem of Evil. What might be a choice given the constraint of this possible world becomes an imposition against the will when constrained only to the universe of all logically possible worlds.