There are those who feel a natural sense of superiority over others, whether it be physical prowess, intellectual capacity, social standing or financial status. A snob, however, generally crosses over the line and exhibits boorish or overbearing behavior towards those he or she views as inferior. A rich person may enjoy the finer things in life, but a snob enjoys watching others suffer because of it. He or she often shows disdain for those who operate in lower social circles.
This is not to suggest that snobs do not have true expertise in their chosen fields, but they often use their sophistication or arcane knowledge to belittle others. These people may not even realize that they come across as effete or snobbish, but others may recognize the characteristics immediately. One might be described as a fashion snob,a wine snob, or a social snob, depending on the circumstances. The distinguishing characteristic between an expert and a snob may be the difference between being opinionated and judgmental. A wine expert may have strong opinions on a certain vintage, for example, but a wine snob would appear indignant if a host opened an inferior bottle.
The origin of the term is a bit murky, and also ironic. One theory is that a ship's manifest often contained a complete list of passengers, along with some notations on their social status, countries of origin and so on. Passengers who were not part of the social elite would be marked as sine nobilitate, Latin for "without nobility". The notation was said to be abbreviated to "snob" in the margins of small guest registers. This practice would ensure that only the most socially recognized guests would be seated near the captain during meals.
Another theory suggests that the term was popularized by students at the elite Oxford College established in England. The students would routinely identify others as fellow students or uneducated local townspeople (snobs). It may help to understand that the Scottish name for a shoemaker was snab, and the English corruption became "snob." It's possible that the Oxford students were using the slang for a shoemaker as social shorthand for all working class townspeople.
Ironically, a term generally used to describe the lower social classes has now become a pejorative term for those who assume an upper class attitude. Essentially, a snob assumes that his or her social, financial, or educational status entitles him or her to be dismissive of others. He or she may come across as an elitist or as a condescending expert, which naturally incurs the ire of others around him or her. Etymologically speaking, however, this is really someone who is playing out of his or her actual social depth.