Every gaming group has a fatbeard: look around the table—if you don't see one then it is probably you.
The fatbeard is a very common creature in the world of nerddom. I am going to be speaking specifically about fatbeards who play role-playing games, however there are also large numbers of comic book, anime, marijuana and Unix fatbeards as well. Any hobby that attracts geeks and doesn't require extensive physical activity will usually have more than a few fatbeards involved.
Each fatbeard is a little bit unique and a lot of people differ on what exactly they think a fatbeard is, so rather than trying to pin down one concrete definition I am going to focus on presenting a general picture of common fatbeard traits. Almost no fatbeards will share all of these traits. In fact, not all fatbeards are fat, nor do all of them even have beards.
The natural habitat of the fatbeard is that of his elderly parents' basement, unless said parents have passed on, in which case the fatbeard might actually occupy the main floor of their former childhood home. Fatbeards rarely leave home as most of them don't really develop much of a career and they almost never marry or date (in fact, many fatbeards are former neckbeards who just got worse). The few fatbeards who do have somewhat of a career usually live in small bachelor apartments that haven't had a visitor in years. It is fortunate that the fatbeard has few visitors, because any visitors he does have would discover a dwelling that stank of cat urine and likely hasn't been properly cleaned in years. Fatbeards usually only venture out of their homes to go to game stores and game conventions, spending the rest of their time posting on message boards and participating in furry porn newsgroups.
In fact poor personal hygiene is the norm for fatbeards, it isn't unusual to hear a fatbeard claim that they are allergic to deodorant, or that people should look past the exterior (smell) of a person and see the person on the inside. Their beards are usually long and poorly maintained, while their hair is usually long and unwashed. Most fatbeards will top all that off with a good case of dandruff along with a greasy pair of hands. Most fatbeards will be attired in a well worn black t-shirt (often with holes in it) with a picture on the front that somewhat relates to their hobby of choice. On the bottom half of things fatbeards usually wear jeans in their early 30s but transition to sweatpants by the time they are 40 (and also, early 30s is about the youngest one can be while still being a fatbeard, as most are over 40).
Fatbeards are more than a little socially awkward and usually have real problems understanding things like personal space or realizing the difference between a conversation and a monologue. The average fatbeard seems to be unable to tell when someone doesn't want to talk to them, and can't properly read clues like how the person may continually back away from them. When that happens a fatbeard just keeps closing that gap, often talking someone right into a corner. You see fatbeards really love their chosen subjects, they obsess over them, far more than the average fan of their hobby. In fact, they don't even particularly care if you are interested in their hobby, and will regale confused store clerks with long intricate stories about their RPG characters and various anecdotes about rules and dice-rolling.
As far as gaming goes fatbeards aren't usually rules-lawyers as much as they are details-geeks. The fatbeard is usually obsessed with making sure everything in their games is run according to the original canon of whatever game or system they prefer, and most will only grudgingly play with newer rules systems or in updated versions of older games settings. Although most are not true rules-lawyers, you will find that the fatbeard will probably have rules knowledge that surpasses the rules lawyer, and will usually have the rules memorized for every version of their favorite games and can often even offer up explanations for the existence of strange rules based on how things worked back in the 1984 version of their game. However, games actually run by fatbeards usually use rules that have been significantly rewritten by the fatbeard in order to "fix" problems with the original system. Their fixed versions usually introduce just as many flaws as they fix though, so they aren't particularly superior.
At the gaming table, the fatbeard is usually a slow player, not even truly considering his action until his turn comes up, at which point he will ponder for ages, and often recite several anecdotes relating to similar situations he has seen in the past. In fact, if allowed, a fatbeard will dominate the entire game with anecdotes to the point where the game basically just stops altogether. Fatbeards often have a hard time keeping a regular gaming group and are very likely to be playing instead with a group of much younger players who haven't been playing all that long. Oddly enough, fatbeards themselves don't tend to form their own groups or cluster together much, preferring instead to spread themselves out amongst the masses.