A popular pen and paper game, D&D has been a major influence on the video game industry.
Dungeons and Dragons is usually played with three or more people, but it is possible for the game to be played with only 2 players. One of the players is the Dungeon Master, or DM. The DM is responsible with describing what is going on in the game world to the players, and then the players act accordingly. The DM is also the final arbiter of the rules, and his/her dominion over the game that s/he is running is absolute. The rolling of polyhedral dice is used to determine many of the outcomes of the actions of the player.
Players take on the role of one of several classes, and can choose to play as one of several races. The main races are Tolkienesque, such as Elves, Orcs, Humans, and Dwarves, but many other interesting races are also present. Planetouched Elemental characters, half dragons, and vampires are all playable, among others.
The stereotypical D&D player is a socially maladjusted loner or loser, who is also a virgin. The stereotype often has pasty skin and severe acne, a speech impediment, and braces.
I, however, have yet to encounter any D&D player that fits this criteria. Therefore, whoever created the D&D stereotype is either a dipshit, or a fucktard.
Nearly all of the people that I know who play D&D are easygoing, laid back, and chilled out. They are aware of the fact that playing D&D isn't necessarily "cool", but they find it enjoyable and think everyone else who judges them for it should fuck off.
Dumbass : You play Dungeons and Dragons...? HAHA HAVE FUN WITH YOUR +1 SWORD AND ELF YOU FAGGOT.
D&D Player : Actually, I have a +3 Vorpal Greatsword. Also, you're a gigantic douche, shut the fuck up and go wack off to Madden or something.