Griswold is a Diablo I NPC, and is located in the center of Tristram, facing Cain and the town well. He is the most useful NPC for most characters, thanks to his convenient location and the fact that he is the only NPC who buys weapons, armor, and jewelry. He is also the only NPC who handles repairs, and he sells a wide variety of normal and magical weapons and armor. Griswold also gives out a few single player quests and quest rewards.
Griswold returns in Diablo II, as a monster. He does not seem to make an re-appearance in Diablo III.