Nethack is a role-playing game. Some might even say Nethack is the role-playing game. It started out (and still is being played) with a text-based display with different characters representing different types of creatures, and that did not stop it becoming a game of great complexity. There is plenty of information about it on the web, written by experts who by all accounts seem to spend at least half their lives in Nethack.
So why am I writing this page? The spoilers (so called because they spoil the game designers the fun of watching players stumble around helplessly for years on end) really tell you everything, such as: "If you #dip a single ordinary long sword in a fountain, are at least experience level 5, and Excalibur doesn't already exist in your game, then there is a 1/6 chance of something special happening to your long sword." As a beginner myself, I was overwhelmed by the information. Though I found what I needed here and there, I decided to write a page which contains what I would have liked to see in the user manual, something which tells you the basic rules of play without too much detail.
I am assuming you are playing Nethack Falconseye, which is a version of Nethack with 3D graphics. The character-based interface is a bit hard on us amateurs.
If you have one of the most common Linux distributions, you will probably have it already on one of your distribution CDs. If not, you can get it from here.
If you compile it from source, you might want to apply this patch. It makes Nethack Falconseye print all messages to the terminal in addition to displaying them in the graphics window. This allows you to review what happened right before you died. Otherwise, you may never find out about why you died, which is in my opinion the most frustrating feature of Nethack Falconseye.
The global configuration files for Nethack Falconseye are located in the "config" subdirectory of the nethack installation directory. They contain comments documenting every option. Your personal configuration file is .nethackrc in your home directory. These options are documented within Nethack. Click on the question mark in the lower right corner of the status bar and select "List of game options" or "Longer explanation of game options".
Two options that everybody recommends turning off are autopickup (which makes you pick up everything you walk over until you break down from exhaustion) and rest_on_space (risks skipping a turn by mistake). Besides you can give a name to your favourite fruit. Copy my one-line .nethackrc if you like:
OPTIONS=!autopickup,!rest_on_space,fruit:avocado
Besides, you may have to mess with the global configuration file jtp_opts.txt to get the sound output right. Enabling both music and sound effects (the default configuration) seems only to work if the sound card supports hardware mixing. In addition, on my system the MIDI player timidity always issues error messages. The simplest way to switch them off is to give a wrong path for the player.
In Nethack, you play a human or human-like being. He/She has a certain occupation which gives him/her certain inherent abilities - an archaeologist can tell which gems are genuine, a rogue is stealthy, a valkyrie doesn't mind the cold and so on. Besides your character has other characteristics which are printed in numerical form at the bottom of the nethack window: Strength (St), dexterity (Dx), constitution (Co), intelligence (In), wisdom (Wi) and charisma (Ch). Constitution determines how long you take to recover from a beating or an illness, intelligence is your ability to learn spells, wisdom helps to cast them and charisma influences how much you pay in shops (yes!).
While these characteristics can be raised occasionally if you survive long enough, the main measure of how far you have got in the game is your experience level (Exp). It determines your maximum hit points and power. These two are probably the most important of your characteristics. The hit points (HP) reflect how much life you have left in you; if they go below zero, you are dead. The power (Pw) is your spell-casting power which is required for (and diminished by) casting a spell. Both can be replenished by resting. The status bar displays both your current hit points and power and their maximal values. When you reach a new experience level, both sets of values are increased.
Another important quantity is your armour class (AC). Unlike the others, a lower armour class is better, as it is added to the chances of monsters hurting you.
Beside your general characteristics (strength and so on) and your current state (hit points and power), you have certain proficiencies in handling certain weapons or casting certain types of spells. You can view these skills with the "enhance" commands (#e) and may be able to advance them when you have practised enough. You will then be told that you "feel more confident" in your fighting or spellcasting skills. Advancing a certain skill is not automatic, because a character cannot become skillful at everything. So there is a tradeoff between different skills, and the player decides what to become proficient at. But enhancing a skill usually does not occur very early in the game, so as a beginner one does not have to do it very frequently.
At every turn in the game, you will decide what you want to do (go somewhere, hit a monster,...), and a random number will decide together with your characteristics what happens (whether you hit, manage to kick a door down,...). The notation for the chances of something happening which you sometimes see in texts about roleplaying is "adb+c". It means that a times a b-sided die is thrown and then c is added. (For C programmers: result= c+a+a*(b-1)*random()/RAND_MAX ;).)
As you walk round the dungeon, you will find things, possibly dropped by monsters you killed (by the way, everything alive except you is called a monster). In the case of scrolls, potions and spellbooks, you will not immediately know what they are, and in fact it can be devilishly hard to find out. This is part of the game (a feature, not a bug). If you want to seriously try to identify much, you will have to look at the spoilers (see below for links).
For a beginner, it is probaly easiest to play a brute-force character such as a barbarian, caveman/woman, samurai, knight or valkyrie. They are strong and tough, and can wear heavy armor which gives them good chances of survival. Besides, the relative hitting power of a broadsword and a dagger is more obvious to the uninitiated than the relative advantages of certain spells.
Once one is a little more experienced at looking things up in spoilers, one might want to try a wizard, healer, priest or monk. These are all characters using magic, and all except the monk are physically weak. I particularly like the monk who can become a martial-arts master, and despite being strong is a capable spell-caster as well. Just don't make him wear any body armor (ring mail, plate mail, etc.), which renders him useless as a figther.
The other characters are generally considered more difficult to play. The tourist is a bit of a joke; the archaeologist can at least dig and tell gems apart; the rogue (thief) is stealthy; and the ranger can become an expert marksman. But they all are none too strong and not good at magic either.
Every character starts out with a pet. Like her/him, it can gain fighting experience, needs food, and can die. There are an infinite number of reasons for staying close to it: It will help you fight monsters. Your god will be pleased if you take care of it. It sometimes steals things from shops for you. It hesitates to walk over cursed things on the floor, so you can test them that way. It is sometimes (for some characters) tougher than you are. In order to take your pet with you when you descend or ascend stairs, you have to wait until it occupies an adjacent square (diagonal qualifies). To find out how well your pet is, move next to it, click on it with the right mouse button and select "chat". Its answer will tell you.
If it has completely and utterly vanished from your level of the dungeon, it may have fallen down a trapdoor (which you don't see unless you saw it going down). In this case you may not want to follow it, since you may end up several levels down. Otherwise, search for it; if you leave your pet alone for too long, it will no longer be tame and you will have lost an ally.