There is a set of typical relations he may maintain in the story. The boy next door is often, but not always, the protagonist of a story. As such, his innocence, sincerity, and common origin will often be contrasted with the cleverness, hypocrisy, and privilege of the antagonist. The boy next door may have a sidekick, who shows somewhat less promise than the boy next door; this will serve to heighten his appeal by contrast. When the boy next door is a sidekick himself, he is often employed to contrast his fresh-faced innocence against the more world wise view of the protagonist. In this scenario, he will often do something well-intentioned but daring which puts him at risk, forcing the protagonist into a course of action to save him.

Often a femme fatale or a girl next door will be pulled between love for the boy next door and her desire for a luxurious life offered by the villain. A boy next door may serve as a love interest for a female protagonist. In this case, he is most likely someone the protagonist has known for most of her life, but in the past couldn't appreciate because of her age. As a love interest, the boy next door is always physically close, yet at the same time detached from the protagonist. He is the sweet boy the protagonist sees everyday, a really great friend, or the perfect boy to bring home to her parents. He is often a virgin. Due to his innocent manner, many erotic fantasies use the boy next door purely as a pretense behind which a real man is secretly very sexual.

Now that the literary concept has been explained, on to something slightly more relevant: A real boy may be described as a boy next door type if he fits enough of the characteristics of one, ie- innocence, sweetness, kindness, naivety, etc.

Boys next door rarely care too much about music or fashion, although they try to keep up. They often wear pastel colors. They are generally just plain sweet. They possibly play some sports, like soccer.

First three paragraphs from wikipedia, last two paragraphs by me, image from deviantart.