What is the difference between Dragonball Z and Dragonball Z Kai?
To put it simply: DragonBall Z Kai is a remake which stays true to the original DragonBall Z Manga. A quick list of the main differences:


No Fillers.
Different theme songs.
Some Different Voice Actors (in e.g: Frieza, Gohan, Android 18).
Less Blood.


Note: Don't watch the censored version, you'll hate it.