blaaf correctly describes the original game, but some versions are more modern (relatively speaking). While using the same primitive text interface, they allow the player to set custom rules.
Simple options include playing without bats and/or pits. Without bats, you won't ever be teleported, making it easier to search the cave. Removing pits means moving is always safe as long as you're not adjacent to the wumpus; personally I think that option makes the game too easy. To increase the difficulty you can add other hazards, including rats that force you to lose a turn or voles that catch and devour your arrows. (Don't ask me to explain that one, I didn't make it up.)
The wumpus can start the game awake or asleep, or random. Each turn the wumpus is awake it may either move to an adjacent room, or go back to sleep, or do neither; the odds of each occurrence are configurable.
You can also set the crookedness of your arrows. This is important because if an arrow ever loops around and returns to your room, it hits you and you die. The easiest setting is "perfectly straight," which means the arrows always leave a room through the hall opposite where they entered. (In a room with an odd number of exits, the arrow stops.) The most difficult is "totally warped", which means that in each room, the shot either stops or chooses a random exit. (It can even go back the way it came, which means any missed shot has a chance of killing you.).
The most interesting option is changing the shape of the map. The default is a dodecahedron with a room at each vertex, and a connection along each edge; that's 20 rooms, each with 3 exits. Other basic shapes are the Platonic solids: an octahedron (6 rooms, 4 exits each), a cube (8 rooms, 3 exits each) and an icosahedron (12 rooms, 5 exits each).
The advanced maps are weirder. They include various sizes of toruses, mobius strips, and a layout called "swiss cheese" that I never did manage to figure out entirely. In these maps, not all rooms have the same number of exits, so your arrow will follow a different path depending on your position.
Finally, there's an option to randomize the rules and map from among the above choices. I always found this the most fun, since it makes the game require far more thought and planning. It's important to explore and develop a mental map of your surroundings, to avoid shooting yourself. If you're on a mobius strip, a straight arrow fired down the length will loop around and kill you-- but a slightly crooked arrow will be safe, and a very crooked arrow fired from the right point will zig-zag all over the place and almost certainly hit the wumpus.
Sadly, my only copy of this game died in the Great Head Crash of '87, along with everything else on the 10MB hard disk of my dad's XT. The only time I have played it since was when I wrote a vastly simplified version for a CS class.