Nerd are people who concerned with mainly things seen as 'uncool' by the general populace. Being extraordinarily tech-savvy, skilled at video games or role-playing games, very well informed in the maths or sciences, are some of the main characteristics. They are 'the smart ones.'
But those of you familiar with the theory of multiple intelligence should know that while nerds may be smart in some regions, their defining feature is being socially 'stupid.' They cannot interact well with anyone, often not with other nerds. This does not make them bad people, nor good. It just means they are not interpersonally intelligent. In some cases, interpersonal intelligence is more useful than technical, so shut up about being proud of nerdiness!
If you are socially inept, it does not mean you are a nerd. Likewise, you can be intelligent and like uncool things but it does not mean you are a nerd. Any definition that says nerds can have social lives and lots of friends and can be popular- they are wrong. Those are not nerds. Those are smart people who are obviously socially intelligent. A nerd requires BOTH aspects of 'uncool' interests AND a lack of interactive skills.
Bill Gates is obviously not a nerd: he is just a techy. But a charismatic and succesful techy.