Taken from Wikipedia.org:

The object of the game is to delve into a dungeon to retrieve the Amulet of Yendor, and perish with as much game points as possible. The player can start out with a different ability set, such as Wizard or Cave(wo)man. The player confronts various monsters: hobgoblins, leprechauns, acid blobs, bats, centaurs, chameleons, dragons, ghosts, imps, trolls, and has weapons, armor, potions, wands, rings and special items to aid in this, e.g. related to fire there is a scroll, a ring, a monster and a wand, and their interplay is to be discovered.

There is time pressure because you die if your food runs out, food is scattered around the dungeon. There is a limit to what you can carry, forcing you to leave valuable items behind. The gold and gems you carry when you die increases your score, but it is heavy too.

The player must enter Hell to recover the Amulet. Entering Hell for the uninitiated just means that “you burn to a crisp”. (In NetHack, Hell is renamed.)

The player encounters special rooms such as shops, crypts, and vaults. Other spatial elements in the game are traps and swamps. As your experience grows, so do your abilities, score and the need for food.

Retrieving the Amulet doesn't guarantee you the high score. After all the Amulet is just a very valuable gem.

Hack_(video_game)#Gameplay