It doesn't have an "exact" meaning, really. It used to be an insulting term that was used to describe people who had poor social skills and were obsessed with unpopular things like computers or video games, comic books, fantasy or science fiction books, or school subjects like science or math. A "nerd" was usually depicted as an unattractive and socially awkward man who spent most of his time programming computers, playing games, or studying. Now, the word often has a less negative meaning, maybe because computers, video games, science fiction, etc., aren't usually seen as unpopular or "weird" anymore. People will sometimes describe themselves as "nerds" because they are intensely interested in a particular subject, a certain TV show, or a book series. For example, the Harry Potter series is often perceived as "nerdy" by people who aren't fans. Some Harry Potter fans might be offended by this, but other fans are perfectly happy to consider themselves "Harry Potter nerds," because they're proud of being interested in something that other people think is weird.

Generally the word nerd is used to describe a person who is very focused on academics or intellectual things in school but not equally focused on other things that are not academic. For example a person who is a nerd is a person who goes to school and studies, then comes home after school and studies and spends all of their free time studying or talking about what they are studying. A person who is a nerd only studies. This person generally does not do anything socially that is not connected to their academic studies. For example if a nerd were invited to a party at where there would be music, food, and dancing. The nerd would say they did not want to go to the party because they would rather stay home and study. If a person who is a nerd were invited to go play sports or go dancing the nerd would say no they would rather study. A person who is a nerd likes to socialize with other people who are nerds because they can spend time studying together and talking about what they like to study. Calling someone a nerd is generally a negative thing to say because it implies that a person only has knowledge of or cares about academic or intellectual things. People also use the word nerd to describe a person who is very interested in and knows a lot of details about a specific subject. For example someone might say " He is a dance nerd, he loves reading about dance and the history dance." Or "She is a basketball nerd, she knows the complete history of the game of basketball and knows the name and position of every player in the league." Also people will call themselves a nerd of a subject that they like a lot and know a lot about. For example " I'm such a language nerd, I love reading about linguistics and the history of language." It is considered less negative to call someone a subject specific nerd (dance nerd, basketball nerd, language nerd etc) then just call them a nerd. But in general calling someone a nerd is not a positive thing.

A type of idiot, who is interested in obscure/strange things. E.g. a computer nerd = a computer geek. "Nerdy" is to be nerd-like. You can say the topic is nerdy; the teenagers dress nerdy, like they are nerds/ strange/weird.