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.