DIABLO
levels
There are 16 levels in the underground area, which are comprised of 4 types.
Levels 1-4: Dungeon
The Dungeon is an ideal starting place for the weaklings. You start here anyway, though. The main enemies are Skeletons, Zombies and those little troll dudes with staffs (I forgot their name, Devil Kin or something). The Dungeon has plenty of libraries (about 1 per level, maybe more), and shrines to aid you on your quest. The rooms are all square as well as on the map.
In the Dungeon you may get the Butcher quest, the Poisoned Water Supply quest, Gharbard the Weak quest, Curse of King Leoric quest, and maybe some more that I haven't mentioned.
Levels 5-8: Catacombs
The Catacombs are pretty much the same as the Dungeon - only harder and different quests, as well as different graphics. The main enemies are the goat people (some with maces, some with bows) and the bats.
The quests you may get are the Bone Chamber quest, the Ogden's Sign quest, the Magic Rock quest, the Archbishop Lazarus quest, and my favorite of those, the Valor's quest.
Levels 9-12: Caves
The Caves are filled with yellow acid rivers, wooden fences, rocks and rock pillars. The main enemies are Poison Spitters (little beasts that spit poison at you), Ogres (big dudes that throws fire rocks at you) and later on, Gargoyles (annoying stone flying things) Ogres that shoot lightning and Demons (big dudes that use inferno on you, make a cool graphic when they die.
The quests you may get are the Black Mushroom quest, the Anvil of Fury quest, and I think that's all.
Levels 13-16: Hell
Hell isn't what you thought it would be. It has lots of chests and barrels everywhere, no doors, nicely planned out architecture, and stairs leading up to the town for those who are on death row. There are Cave Vipers, Demons, more of those Goat people, and towards the end the most annoying enemy in the game comes out. Picture a fairy, dressed in red or blue, that uses Blood Spirit or whatever it's called on you, always runs away at your speed when you come too close, and hangs out with about 30 friends. YES, ANNOYING! The only way I found to beat them is with magic (or a bow, but that's less effective). Just keep using firebolt and lightning and stuff.
I don't think you get any quests this late in the game.
Final Level: Diablo's Lair
This is the last and probably the hardest level in the game. Diablo himself resides here, protected by his many minions (how cool does that sound?). To get to him, walk around the level until you find a lever or something. It will open up a new area, if there isn't one open already. Keep doing that until you unlock the final area. Diablo will come out, say something, and attempt to kick your ass.
Infravison is good when trying to unlock the areas, as there are a shitload of enemies just waiting to banish you to, well, hell.
Once you finish it, watch the ending sequence, then go out and buy Diablo: Hellfire and Diablo 2.