There are many definitions of "Nerd" by many people. Some see them as highly intellectual people, others see them as scrawny or obese people with bad acne and thick glasses.
Truth be told, truthfully they are neither. Though, all opinions are based on the outlook of the person giving them.
To me, nerds are generally the smarter people who have a large interest in one specific hobby or hobbies, and are quite knowledgeable of that hobby. The common stereotypes of a nerd's hobbies are computers, gaming, and things like Action Figure collection, D & D, and Warhammer.
Often enough, people believe nerds to be unsociable, unliked, and either rudely proud of their hobbies and/or their intelligence (thus creating jealousy) or shy.
In truth, nerds come in "all shapes and sizes". There are short nerds, tall nerds, skinny nerds, fat nerds, girl nerds, nerds with glasses, nerds with weird socks, and etc.
Also somewhat often, you can't even tell who’s by definition a nerd until you get to know them. Nerds can fit into life with their hobbies, or stick out in one way or another. Nerds can be socially inapt, but that’s because of the nature of their hobby.
Take Bill Gates
for example. He is (or was) a nerd. He's smart, had a hobby which he particularly was knowledgeable and liked very much (Computers), and nowadays is very successful because of it. He is one of the richest men in the world, and grew up being one of the most frequently insulted stereotypes.
What I mean by that, is that often enough "nerd" is used as a direct insult to someone who’s either very smart or is knowledgeable about a particular hobby he likes. Very often people who are called a nerd are called that because the person who said it was jealous of them in some way.
Nerds often become the rich and successful, and your boss can easily be a nerd. Nerds created and evolved a lot of technology. Computers, cars, engines, TVs, stereos, and much more have had nerds involved.
Though nerds can also be overly-obsessive, and will not become rich and/or successful. Most that has been defined about is protecting nerds, though in reality there are many who can take it too far. Some can be consumed by their hobbies. A recent example is World of Warcraft
nerds. Some play every now and then, but for some others they live in the World of Warcraft.
The generalization that nerds are ugly, unsocial, and have thick glasses can be totally true. Some do follow that generalization, but that is because of their nature. They might not care about what others see them as. Most nerds aren't like that. I'm a "nerd" and unless you followed me around for a few days you would have no idea. I'm social, I have friends, and I play sports. Nerds are people, like Jocks or Emos.
Some nerds should also be respected. Why? Well, the rich and successful ones should be. They either used their intelligence to get them rich and successful, or even turned their hobbies they loved into a career. Another reason they should be is because they aren't afraid to be themselves, for the most part. They aren't afraid that others will think they are a "nerd" or "loser" because they love their hobbies.
Nerd can also mean highly an intellectual person, of course. Those people who enjoy doing their homework and are in the pursuit of more knowledge can easily be classified as nerds.
Guy 1: Look at that nerd over there! His glasses are so thick I can't tell his eye colour!
Guy 2: I wouldn't say that if I were you, one day he'll probably be your boss or something.
Girl 1: Yea, my boss is a total nerd and makes a tonne of money!