According to Church teaching, there are two "pains" associated with Hell, poena damni and poena sensus.
Poena damni, the pain of loss, is the worse of the two pains. It is the knowledge that one's soul is completely separated from God, and that one will be denied the beatific vision for eternity.
Poena sensus, the pain of the senses, consists of the physical torment suffered by the damned. Most of history's theologians regard a real, material fire to be what causes this torment. Some other theologians regard "hell-fire" metaphorically, and believe that it is not fire as we know it causing torment to the souls of the damned, but something metaphysical beyond our understanding. As far as I know, the Church has never official endorsed or censured one side or the other.