Entering the dungeons you find either a kitten or puppy waiting for you. The quest assigned is different with each different character type. I forgot one charactertype, the rogue. There are shops and vaults to be robbed and many cops to kill. Sacraficing offerings to your alligned god might get you artifacts.
Artifacts are special weapons with special powers.
Each game the dungeon is different. You can play the game many times and never play the same game that way. The support newsgroup for nethack is rec.games.roguelike.nethack.