If something is nocturnal, it belongs to or is active at night. That includes vampires, owls that like to hunt by moonlight, and that roommate who stays up playing video games until the wee hours of the morning.
The adjective nocturnal comes from the Late Latin nocturnalis, which means “belonging to the night." You've probably heard of nocturnal animals, like bats and fireflies, who sleep during the day and come out to play when the sun goes down. The opposite of nocturnal is diurnal, meaning active during the daytime — but you probably won't be using that word much unless you're a zoologist.