The title describes what the main characters are: a bunch of kids who get together at breakfast time because they've all been sentenced to detention. But will they continue to get together for breakfast in the future, without being sentenced to detention? Or will they get sentenced to detention again, because Brian's the only one who bothered to complete the assignment Vernon gave them and write an essay? These questions are left hanging at the end of the movie.

At the beginning of the movie, the five kids aren't really a cohesive "club" of any sort. They're just a bunch of people who all happen to have been sentenced to detention. The movie details the process by which they develop this solidarity with each other and really do become "The Breakfast Club" of the title.

Once they've seen into each other's lives and gained understanding, they all start to relate and identify with each. In the last lines of the movie, Brian signs off by reading the essay he wrote to Vernon:

BRIAN (voiceover): But we found out that each of us is a brain, and an athlete, and a basket case, a princess, and a criminal. Does that answer your question? Sincerely yours, The Breakfast Club.

Vernon's question was, "Who do you think you are?" That's supposed to be the prompt for the essay—and it is. Brian actually does a great job of answering it, giving a thoughtful answer that goes way beyond what Vernon was expecting.

By referring to themselves as "The Breakfast Club" he highlights the unity and togetherness they've developed through the day, and the fact that they don't view each other as being separate species anymore. Now, whether Vernon is going to enjoy this unique approach to the essay is another question in itself….