Neckbeards are, unsurprisingly, the
lowest greatest most annoying forms of human life on Earth. They generally appear as a result of failed shaving skill rolls. They are widely believed to be a typical part of appearance of any nerd or roleplayer. This is a primary reason why users frequenting /tg/ are nicknamed "Neckbeards" as well. They are also called fa/tg/uys. The only real difference is that you do not need to frequent /tg/ to be a Neckbeard (or have one).
According to legend, great power can be gleaned by the possession of a neckbeard. Tales tell of a fa/tg/uy who had near-godlike levels of nerdrage, that were a direct result of his massive neckbeard.
A more plausible theory claims that neckbeards are in fact a form of clitoris that has evolved to be independent of humans and have become rotten and repulsive. Once a month, they get together to inject their DNA into their fellow neckbeards in order to procreate. Those with neckbeards grow them because no one will ever love them and thus, there is no longer a desire to groom due to the acceptance of their inherent repulsiveness.
The oldest Neckbeards are called Grey Beards, and are old. Hence the space in their name, out of reverence. Banhammer40kun Is known to have a large neckbeard.
It should be noted that the internet has, in recent years, been using the term "neckbearded" as a derogatory slur against antisocial, socially-incompetent basement-dwellers. This usage is erroneous, and most likely originated from some moron seeing a fa/tg/uy use the term "neckbeard" outside of /tg/, and decided to pick up its usage without knowing what it meant, much like a monkey copying a human he saw writing by banging a twig on some bark. While /tg/ can be both antisocial and basement-dwelling, the term "neckbeard" has always been used affectionately, and being called a Neckbeard is often a sure sign that /tg/ finds you similar to themselves (tolerability, however, is another matter).