How Do I Play Dungeon?
This answer is about the mechanics of entering commands and reading the results. For basics re what should you be trying to do, have a look here.
Dungeon's a textual virtual world. You "play" it with words.
We'll use the screen shot below to explain how our GUI works. It's a picture of the Dungeon interface for players. (It's been shrunk to fit here: the real one fills your entire browser page.) There are four parts to the screen:
1. The Character panel on the left gives you a quick graphical overview of your character's current score. Each achievement will also be listed.
2. The Game panel in the center displays messages sent to you by the game. This will include descriptions of the place where your character is currently located; descriptions of the things or people you look at; descriptions of events that happen around you; and outcomes of the commands you type. For example, you may type "look door": this is where the description you asked for will be displayed.
3. The Chat panel on the right displays messages sent by other players. These could be messages intended for everybody; messages intended just for the characters currently located in the same place as you; or messages intended for you privately. They'll be color-coded by type so you can tell them apart.
4. The Command panel at the bottom is where you enter the commands which cause your character to take actions within the game world. We used the example "look door" above: this is where you'd type that command. Just hit the enter (or return) key to send your command to the game.
That's it. Simply type your command, then press enter; and read the messages in the game and chat panels. Not too tough, eh?
The Command Reference lists every available command; you can find it here. You should read the section titled "Basic Commands" before starting. You'll have more fun if you learn the other commands eventually too; but you can do that as you go along.