The first reaction I get when I say "I play Dungeons and Dragons" is "wow!". Unfortunately, the second response is "What IS Dungeons and Dragons?". The third is "Can you teach me?".*
I can teach you.
It takes a really freaking long time to learn all the rules. Why? Because there's a lot of rules. And a lot of those rules are complicated. And a lot of the complicated rules use a lot of different, foreign, strange jargon.
So, in order to learn D&D, you're going to have to learn to speak D&D.
So here's a short guide for some of the common words.
D. D is an abbreviation for the word "dice". You use it with a number, with the D usually coming before the number. For example, D20 means a twenty sided dice.
D12: This dice looks somewhat like the D20, only it's... Well... got twelve sides, not 20. The twelve sided dice is used mostly for damage rolls, for weapons such as the Greataxe. This is one of the least used of the dice.
D8: the D8 (not the date), is another one of the damage roll dice. It's much more commonly used than the D12... I'd say about a third of the weapons you can use have the D8 as the damage dice.
D10: The D10 is also a damage roll dice, but it also has it's own special thing. When you couple it with the 10s dice, (which have the numbers 10, 20, 30, 40, 50, 60, 70, 80, 90, and 00 on it), and roll them together, they become the percentage dice. These are really helpful when you're the DM, and you're choosing things using a percentage table.
D6: the D6 is another very commonly used dice. After the D20, I'd have to say it's the most commonly used dice. They're used for rolling up character sheets, for attack rolls, and for stacking up into tiny towers when the other players are taking way too long deciding what they're going to do.
D4: the D4 is yet another damage roll dice... That's pretty much all you use it for. :P
DM: The DM is short for The Dungeon Master. The DM runs the game, and plays the NPCs, the monsters, and describes the rooms. They're also the ones who'll take you literally if you say "I jump off a cliff!" even if you're only joking.
AC: really easy one here. the AC is your armor class, and it shows just how tough you are to hit. At least in the 4th edition, you really want this to be high. If you have an armor class of 15, then only an attack roll that's 15+ will hit you.
HP: another easy one. Hit Points. These tell you how much health you have.
Character Sheet: the Character Sheet is the place where everything about your character is recorded... The AC, HP, your weapons, equipment, languages, race, class, etc., etc., etc.
Tiles: Gaming Tiles. These are the things that make up a little map on the table/counter/floor where you're playing. Your figurine, which represents your character, moves on top of the tiles, which gives you a visual as to where your character, other PCs, and the monsters are.
In Game/Out of Game: when you're "in game", it means that you're in character, and you're speaking, or describing your characters actions, without asking the DM questions, or talking about not game related subjects. When you're Out of Game, you're not in character... You can be asking questions of the DM, such as "But how can he do that?!", saying stupid things such as "I jump off the cliff!", being actually sensible by saying things like, "I have to go now", or you could just be cluttering up the gaming table with annoying senseless chatter. You usually want to be IN game.
PC: no, not the computer! An PC is a Player Character... Which is pretty much self explanatory. If a character is a PC, it means that a fellow gamer is playing it. This is opposed to the NPC, which is when the character is being played the Dungeon Master, who is not considered to be a player.
Race: Pretty self explanatory. This is things like, Elf, Human, Gnome, Halfling, Dragonborn, etc., etc., etc.
Class: Again. Pretty simple. This is what you are, in terms of your job. Example: Rogue, Fighter, Paladin, Wizard, etc.
*Unless of course it's another D&Der. In which case I usually get either a "That's sooooo cool!", or, if it's a boy, "But... I didn't think girls play D&D! O_o"
So, that's it for today... I think... I'll have more D&D explanatory stuff up soon, I think. Probably stuff about the Character Sheets, or the books... Possibly more jargon (D&D has a LOT of jargon).