John Green is an award-winning YA novelist and a famous Internet vlogger (along with his brother, Hank).
Green has written several novels for young adults, securing the Printz Award for his first book, Looking for Alaska. He's also worked on several books in collaboration with other young adult authors, including Will Grayson Will Grayson with David Levithan, and Let It Snow with Maureen Johnson and Lauren Myracle.
On the internet, he's probably best known for being one half of the Vlog Brothers. That, or the writer behind the interactive novel/riddle game thisisnottom. He is also currently giving lectures on world history with the program Crash Course.
Before becoming a full-time writer, he worked as a book reviewer for Booklist and as a chaplain at a hospital.