A nerd is essentially an ugly, anthropophobic luser*, often fat and / or a basement-dweller, who is obsessed with and quite knowledgeable about a particular, non-mainstream hobby. In practice they are essentially a subset of the geeks, except that they are even less socially adjusted. Thus, unlike geeks, nerds are notoriously easy to pick out of a crowd — one need only listen for the person who is prattling on endlessly about a subject of little interest to anyone else, or who only wears t-shirts that they purchased at last week's convention. If you are reading this article, then you are a nerd. This is doubly true if you have ever edited a wiki... although ARRSE wiki Sysops are a breed apart! Remember this or we'll suspend your account, go round to your house and give your wife something to hang her towels on!
The Nerd Kingdom
The term "nerd" is merely the genus of a vast variety of social degenerates; there exist, under the broad categorization of the term, many species of nerd that are determined based on collective nicknames. A nerd who is obsessed with masturbating to pictures of scantily-clad elves in their parents' basement is a role-playing nerd, for example. Nerds that spend all their time immersed in a particular canon work or fandom are fangirls, even if they are male. In this latter instance, fangirls are divided up even further based on their chosen fandom — Star Trek nerds are called Trekkies, Lord of the Rings nerds are called Ringers, and so on.
Another major nerd species includes nerds who compulsively update their Blogs, although fate has been kinder to them and they tend to go by the moniker blogger.
Warhammer Nerds are like any other nerds in the way that they have no hope of ever having sex, but they are also different because they contribute to society. Warhammer Nerds contribute to society because they have high paying jobs due to their need for money for Warhammer where each plastic tank costs at least £70.
This makes Warhammer different from other nerd activities like World of Warcraft where 40 year old virgins sit in their parent's basements not doing any work for society. Warhammers Nerds work for the greater good by leaving their houses and getting jobs and rapidly advancing the career ladder to get £300 for a plastic artillery battery.
In some cases, it may be fair to group re-enactors in this grouping.
Avionics Technicians are usually Warhammer Nerds.
There is nothing worse than coming across a Political Nerd. Political nerds believe they are unquestionable political pundits despite a lack any knowledge of the topic at hand. Often the extent of their political knowledge is from a post by someone on a forum or a video created as a joke. ARRSE is stuffed with this type of nerd.
A Disturbing Truth
Nerds can be disturbingly successful in their chosen field (ie computing). Bill Gates of Microsoft fame is a nerd. You can find yourself working for a nerd. The pencil pusher you need assistance from may be a part time assistant in Games Workshop (producer of Warhammer). So while you can and should mock the nerd ... just remember that the Geeks probably have inherited the Earth.