For much of society, the words geek and nerd are synonymous. It seems to me that this is not the case in geekdom, for lack of a better term, where the two words refer to two similar sorts of people with some fundamental differences. It seems to me that most geeks perceive a difference between geeks and nerds, and I have noticed the words do tend to be used differently. Indeed, I consider myself a geek, but in no way do I conisder myself a nerd.
To get an idea of the differences between geeks and nerds, it might be a good idea to look at the history of the two words. The word geek stems from a Scottish dialectal word, geck, meaning "fool," itself deriving from Middle Low German gek. The word geck appears in Shakespeare's Twelfth Night: "Why have you suffer'd me to be imprison'd. And made the most notorious gecke and gull that ere invention played on?" A variant spelling, closer to modern geek, also appears in the Bard's Cymbeline: "To taint his nobler heart and brain, with needless jealousy, and to become the geeke and scorn o'th' others villiany?"
By the 19th century the word geek was being used in the United States as a word for an offensive or undesirable person. It was not long before it also attained one of its most famous senses, that of a sideshow performer who would commit bizarre acts, such as biting the heads off birds, swallowing live bugs, or pounding nails into their skulls. Geeks were considered the lowest form of life even in carnival sideshow circles, and this was perhaps for good reason. Many geeks were either drunks who would literally do anything for drinking money, while others were simply deranged. Still, there were yet other geeks who were simply skilled performers, although the skills they possessed were outré to say the least. From an undesirable or offensive person or a very specialised (to say the least) sideshow performer, the term geek took on the meaning of someone who is intensely devoted to something outside the mainstream. The keywords here are "intensely devoted" and "outside the mainstream." One can be intensely devoted to the movie Gone With the Wind, but one cannot be a Gone With the Wind geek. The reason is because Gone with the Wind is part of the mainstream. One can have a casual interest in Star Trek, but he or she would not be a Star Trek geek. While Star Trek arguably lies outside the mainstream, it takes extreme devotion rather than casual interest to be a Star Trek geek.
If the modern use of the word geek as someone with an intense devotion to something outside the mainstream stems from its use for a very specialised sideshow performer, I think this might say something about the nature of geekdom. Quite simply, geeks are geeks by choice. Arguably, except for those who were hopelessly deranged, sideshow geeks could have chosen another profession, yet they did not. By the same token, modern geeks do not have to be slavishly devoted to Lord of the Rings or anime or computers, and so on, but they choose to be so. Indeed, they choose to do so even though this may well place them at odds with the rest of society's tastes in books and movies. Quite simply, a geek is a nonconformist with an extreme devotion to something outside the norm.
While the origins of the word geek are more or less well documented, the origins of the word nerd are more obscure. In fact, the word seems to be unknown before 1950. As to how or where the word originated, no one can say for certain. A word nerd appears as the name of a creature in Dr. Seuss's If I Ran the Zoo: "And then, just to show them, I'll sail to Ka-Troo And Bring Back an It-Kutch a Preep and a Proo A Nerkle a Nerd and a Seersucker, too!" In the book, the nerd is a very thin hominid. Given that Dr. Seuss does not tell precisely what a nerd is and the illustration tells us little more than nerds are thin, it seems doubtful to me that the modern slang term derives from If I Ran the Zoo by Dr. Seuss. Yet another etymology maintains that the word nerd could derive from a variation on the last name of ventriloquist Edgar Bergen's dummy Mortimer Snerd. The one problem I can see with this derivation is that the word does not appear before 1950, a time when Mortimer Snerd's fame was not what it once was, making it unlikely that a variation on his last name would enter the language as a slang term.
Regardless, the word nerd may have been an established slang term by the late Fifties. It appears in 1957 in a regular column devoted to teenage slang called "ABC for SQUARES" in the Glasgow Sunday Mail. There it is defined simply as "a square." It could well have been an American slang term, as the word "square" itself was. Of course, a square is someone who is out of touch with the latest trends, someone regarded as coventional, a rigid conformist. If nerd was an established part of American slang by the late Fifties, that might explain why philosopher Timothy Charles Paul Fuller adopted the term to describe a reclusive intellectual with poor social skills in the Sixties. It would appear that this is the meaning that stuck, as by the Seventies nerds were regarded as intelligent individuals who have poor social skills, a meaning it has retained to this day. While geeks are nonconformists who have chosen their lot, nerds are simply socially inept individuals with little choice in the matter but to improve their people skills.
This brings us to what I feel to be the fundamental differences between geeks and nerds. A geek is simply a nonconformist who chooses to be extremely devoted to things that lie outside the mainstream. It is not that a geek lacks social skills or cannot get along with people, it is simply that he or she chooses not to conform to society's expectations. Just as sideshow geeks chose a profession that was outside the mainstream, a geek chooses pursuits that are outside the mainstream. On the other hand, a nerd lacks social skills. He or she simply does not know how to interact with people. In many respects, a nerd is a "square" in that he or she is to a degree out of touch with society. To put it more bluntly, a geek may well have an active social life. He or she may date, go to parties, and do everything that other people do. A nerd might well spend his or her evenings in his parents' basement watching old reruns of Lost in Space. While a nerd may be hopeless devoted to, say, Farscape in the same way that a geek may be, he or she does not have the social skills to have much of a life beyond fandom (often not even that).
In fact, the implication of intense devotion inherent in the word geek seems to be another area in which geeks differ from nerds. I have observed that the word geek is often used where we might expect the words fan or devotee, i.e. Lord of the Rings geek, computer geek, and so on. The word geek appears to me to differ from the words fan and devotee in that it implies a higher form of devotion (one bordering on obsession, perhaps) and, as pointed out above, it implies a devotion to something off the beaten track (Star Wars, heavy metal music, et. al.). As near as I can tell, the word nerd implies no such devotion. It is fully possible for one to be a nerd and not be devoted to anything.
To get a better idea of the differences between geeks and nerds, we might well look at pop culture. To me the perfect example of a geek is Q from the James Bond movies. Q is clearly devoted to the creation of unusual gadgets. He will gladly explain to 007 what each gadget can do and chides him when he doesn't bring those gadgets back intact. Yet, Q gets along quite well with people. In fact, he seems to have a high degree of social skills. The perfect example of a nerd would be the Comic Book Guy from The Simpsons. Overweight, unattractive, and wholly abrasive, the Comic Book Guy simply rubs people the wrong way. In fact, one wonders how he stayed in business all these years, given the fact that he could well drive off all his customers with his personality! While he has a devotion to comic books that a geek might well have, he lacks the social skills that a geek would have as well.
Given that I do have social skills and I do have a life beyond the computer, Lord of the Rings, Star Wars, et. al., I believe that I am a geek rather than a nerd. Geekdom is something I have chosen for myself. I can and do interact with "mundanes" and talk about such normal things as sports and the weather, topics which may be out of reach for the typical nerd. Unfortunately, for the most part I am not sure that society at large realise the terms refer to two different types of people. While the average person may look at me and know I am not a nerd, at the same time they probably do not realise I am a geek (thinking the two words synonyms). Whether using the terms correctly matters beyond geekdom, well, that is a topic for another time