"What defines upstream? KDE, GNOME, Apache, even the Linux kernel are upstream for a Linux distribution, and even a totally different distribution altogether may be considered upstream (for example, Debian is considered upstream for Ubuntu). These upstream projects are the ones that write most of the code that goes into powering your free software operating systems, and to a distribution, they are what makes the whole thing possible. There is a fine but delicate process that happens to keep all of these projects all running as smoothly as they do. Distros have been known to fail due to making too many changes to the source without the cooperation of their upstream partners.
"You can trace the path even further upstream: KDE, which is already upstream for a distribution, depends on code that comes out of a wide variety of library projects, such as the popular Qt toolkit library..."
