Dragon Ball Z is the second part of this popular series. This part is the one which was the cause of the boom of its popularity, which increased astronomically. For example, the first part saw three films based on its argument... Well, Dragon Ball Z has been the base to 15 films !! This fact alone enables to see the enormous popularity of Dragon Ball. We can find manga, special colored manga based on the films, lots of CD's, an illimited number of card collections, videogames for Nintendo, Super Nintendo, Megadrive, Playstation, Saturn and Nintendo 64, posters, figures, key rings, candies, pens, and so on, all based on Dragon Ball Z. There is also a 7 volume encyclopedia where all kinds of information about Dragon Ball Z can be found. The manga has been translated into lots of languages, as the Chinese, Korean, Catalan, Italian, Spanish, French, Malay, etc... This is for the manga, but I cannot imagine into how many languages has been translated the anime series...
Why this series has achieved such an incredible popularity? Dragon Ball Z keeps telling the adventures of Son Gokou and his friends, but the humour that we could find in the first part disappears now. There is much more action, there are lots and lots of energy attacks, and a fight is not a fight without them. In Dragon Ball Z appear for the first time the saiyajin, a mythical alien race. Here we discover that Gokou is a member of this race (he didn't know it, and now we can understand why he has a tail and why he transforms into a big monkey during full moon nights).
Besides, in Dragon Ball Z the family grows because some characters have children, like Son Gokou, who has two sons with Chichi (Son Gohan and Son Goten) and Vejita, who has a son with Bulma, named Trunks.
As the series goes on, Goku keeps winning more and more enemies, which are more and more strong, and he himself gets more and more strength. An example: At the end of Dragon Ball, Gokou is the strongest person in the Earth. Well, in Dragon Ball Z some extraterrestrial characters come into action, at first the saiyajin Vejita and Nappa, after that, the strongest being in the universe, Freezer. When Gokou wins Freezer (and then Gokou is the most powerful in the universe) two androids created with the purpose of killing him come, and then a bioandroid even strongest, Cell, comes into action. Then, when Cell finally dies, Buu, a creation of an evil wizard, arrives. Of course, there is no plot left, it consists only in a repetition of the same. But DBZ has the quality of keeping us hooked on it by its action and fantasy.
It was precisely this fact, the evident poorness of the plot and the dificulty in finding an exit to this repetition of "now he wins the bad boy and then an even stronger one comes" what made Akira Toriyama take the decision of leaving the drawing of the series to make other things, other series that, I am sure of it, will also have a great quality.
Perhaps one of the things which draw more attention is the appearance of the supersaiyajin. The supersaiyajin are saiyajin with have achieved such an state, during which their strength increases incredibly, their hair turns into blonde ans sharpens and their eyes turn into blue. During the series, Gokou and the other saiyajin after him achieve level 1, then level 2 (during which the hair gets even sharper) and level 3 (which only Gokou achieves and during which the hair is still blonde but very long).
During the last part of the series the fusions enter into action. The fusions, as its name indicates clearly, consist on a fusion between two saiyajin of an equivalent power level. This was made in answer to Buu, a monster so strong that nobody could beat him. The solution was to join two characters into one and then their force would double. The first ones who used this technique were Trunks, Bulma and Vejita's son, and Son Goten, Gokou and Chich's son. These two form Gotenks. After that, Goku and Vejita fusion, creating a character whose Japanese name is Vejito, but whose Catalan name may be Vegeku (in the manga), Gogeta (in the TV series) or Vegerot (also in the manga.)
To finish with, I would like to recommend this series to anybody, but I personally prefer the first part, because of its better plot, and its incledible sense of humour. Well, Dragon Ball Z is a shameless repetition of the same situations. The first half is not so unimaginative made, but the second half insists always in the same awful and boring situations of fights during 30 episodes or more...