Reading the Raspberry Pi Blog, Jessie was released before the R-Pi:0 which should have went smoothly like Wheezy did for the R-Pi 2 but it didn't.
 
Thus Jessie has been re-released with updates for the R-Pi:0, a few bug fixes and additional software. There are a few issues that are still apparent that will cause issues with the R-Pi:0, notably the Overclocking configuring - changing it will slow down the R-Pi:0, not speed it up. This will be fixed on the next release of Raspian.
 
KDE and Gnome were also added as installable GUIs instead of LDE (Light version of KDE) for Jessie on this update as well.
 
You can do an update to get the bug fixes but not the additional software. You need to install the additional software one at a time in by hand or download and reinstall Rasbian on your SD Card all at once.
 
Personal Opinion: Looks like Jessie was released prematurely and was rushed to get it out... D'Oh!
 
Personal Question: If they put in KDE, will it have KDE Office? Don't know.
 
 
 
The Blog article is here:
https://www.raspberrypi.org/blog/latest-raspbian-update/
 
Overclocking Issue:
https://www.raspberrypi.org/blog/latest-raspbian-update/#comment-1249865
 
Jessie Lite (Raspbian without the GUI, great for the Model A/A+ with 256MB)
https://www.raspberrypi.org/blog/latest-raspbian-update/#comment-1249886
https://www.raspberrypi.org/downloads/raspbian/