The first game in the series lets you play as either a warrior, sorcerer, or rouge as you fight the demons of hell through 16 randomly generated levels. In addition to fighting demons, you can also take on additional quests from villagers scattered accross the map.
Diablo II
The game plays in four acts, one act in each major town. Each town faces destruction by a powerful evil force, which you must destroy. You must fight all of these evils in each major town before you can face Diablo, the ultimate evil. The second game in the Diablo series is a major improvement over its predecesor. The game is four times longer than the original, with two new character classes, 3-D graphics, day/night cycles, weather effects, and a ton more weapons, armor, items, and spells.
Diablo II: Lord of Destruction
Because of the success of Diablo II, Blizzard decided to release an expansion pack. The expansion introduces two new classess, the male Druid and the female Assasin, each with there own unique skills. There are more monsters than in the previous game, powerful sets and class specific items, new Horadic Cube recipes, more interactive environments, and a brand new fifth act featuring six quests in the Barbarian Highlands.