Saiyans
The best fighters in Dragonball were saiyans basically. They are probably the strongest
race in the universe. They can become stronger after each battle. They come from the
planet Vegeta, which was destroyed by Frieza and there are only a handful of them left.
They have the ability to go Oozaru on a full moon and they can turn into Super Saiyans
which makes them even more powerful then they originally were.
Human Z fighters
They're not even close to the power of the saiyans, but they are much more powerful than the
average human. Krillin falls into this catagory as the strongest human followed by Tien, and Yamcha.
Normal Humans
Characters which basically dont have enough
fighting skill to help out the z fighters but they do play a big role in the series such include Mr.Sataan and Mater Roshi.
Namekians
These green beings come from the planet namek. They have the ability to regenerate lost
body parts but they arent too great of fighters, at least most of them. There is an exception
for Piccolo. Some nameks like Kami, or Dende were key characters in the dragonball
series.
Androids
The androids were created by Dr. Gero and are very powerful. They are robots who act
and look like humans. They dont loose their power levels and their power cannot be
detected.