Dragon Ball Z is the best of all Dragon Balls in my opinion. This sequel tells us the whole Goku's history. We learn more about the Saiyajin civilization, how they where and how they get stronger.
In the first part of Dragon Ball Z we meet 3 Saiyajin (Vegeta, Raditz who is Goku's brother and Napa). Also we see Piccolo's history and a lot about the namekians. Goku have to defeat them because they want to destroy the earth, but all of them where Freezer's servants. This is why, Goku get stronger and become the Legendary Super Saiyajin, his first transformation.
There's also more enemies, like the androids 16, 17, 18 and Cell, the strongest one. A young boy from the future, Trunks, travels in the time to cure to Goku, who has a serious heart affection. Trunks is trying to learn how to defeat the androids, and we realised that he is Vegeta and Bulma 's son.
Goku's son, Gohan (named after Goku's grandfather) is the one who defeat Cell. Here is when the saiyajins get the second transformation, the Super Saiyajin 2.
Then appears a worst enemy, Majin Boo, and for the first time we can see another transformation, Son Goku as the Super Saiyajin 3. But to defeat to Majin Boo, they have to use the fusion, because he is to strong.
Those where the 3 main parts or sagas in Dragon Ball Z. As we are used to, there are great fights, funny situation and a lot of action.
Dragon Ball Z, despite of being long (291 episodes) it's on of the best things that you can see in Animated Japanese Series (Anime).