They will re-watch episodes and movies and re-read books multiple times and follow the series closely. They will buy all the related merchandise, often collecting and keeping it in mint-condition. They will attend conventions, commonly while dress as their favourite characters.
There is a social stigma towards nerds, particularly in schools. Since nerds tend to obsess over particular things, usually things that are unpopular, they will often go unaware of current popular trends and are view by others to be uncool for their alternative interests.
Nerds are commonly viewed as very intelligent due to their hunger for information.
"I here he also goes to conventions all the time and will dress up as characters from movies"
"What a Nerd"
Often mistaken for Geeks, who aspire to become nerds, yet lack the intelligence, and end up giving nerds a bad name due to their poor social skills.
Non-nerds are often scared of nerds, due to their detailed knowledge, and therefore seemingly high levels of intelligence - and subsequently denegrate them as much as possible as often as possible.
Nerds exist covertly within the fabric of society, often choosing to 'nerd it up' in private or in the company of fellow nerds. It is for this reason they are feared the most - unlike geeks, who are easily identified, nerds can only be found out when casual conversation reaches a subject that they like nerding.
Nerd: "Let's bash this nerd"
Nerd: "That'll teach you to go playing with my tesla coil"