Dragonball Kai is basically a remastered version of Dragon Ball Z with HDTV episodes, featuring updated opening and ending sequences, and a rerecording of the vocal tracks by most of the original cast.
The Dragon Ball Z anime series originally ran from April 1989 to January 1996 in Japan. However, since Toriyama was still serializing his manga at the same time in shorter weekly installments, the Dragon Ball Z anime expanded its story by inserting additional content.
The new Dragon Ball Kai remastering will be an "Akira Toriyama original cut edition" that will quicken the story presentation and adhere more with the manga's story.
These episodes were also re-edited to more closely follow the manga, resulting in a faster moving story, and damaged frames removed. As such, it is a "new" series created from the original Dragon Ball Z footage.