The book originates from Berklee School of Music, where the students there made their own chord charts for all the jazz standards they were required to learn. The copies made were uncopyrighted and so were illegal. However they were passed around, copied and copied by musicians.
The Real Book produced by the Hal Leonard Co. is fully legal and has secured all the relevant copyrights. It also includes consistent use of chord symbols throughout the book, clear page layouts, clear type and organization of songs into alphabetical order.
In tribute and acknowledgement to the 5 editions that came before it, the Real Book available now is called the 6th Edition Real Book.