Dragonborn are decendents of the great dragons, the oldest of all the mortal races. Dragonborns are the offspring of dragons that took on human from and mated with humans. After time the mating of dragons and humans was looked at as a dishonorable thing and is rarely done anymore. The decendents of those pairings however are a noble and honorable race.
Other information is the same as in the Player’s Handbook.