Humans

Humans (sometimes referred to as just Earthlings) are the main inhabitants of the planet Earth in Dragon Ball. They share the planet with other intelligent lifeforms, including anthropomorphic animals. In fact, they are lead by an anthropomorphic dog named King Furry.

With the exception of the uncommon hair colors and occasionally odd features (ranging from longer lifespans to werewolfism), Dragon Ball humans are identical to their real-life counterparts. Humans in Dragon Ball seem to be able to utilize or be affected by certain elements of magic. Akira Toriyama himself has admitted to lack of forethought or planning when it comes to the details of Dragon Ball's story, which accounts for some of the strange occurrences.
Humans have a large military, which has appeared through out the series, mostly consisting of ground troops, tanks, fighter jets, and large naval ships. Earth's military usually appears when attempting to defeat many of the villains that cause massive damage to the Earth (examples being Cell and Majin Buu), but are always easily wiped out without causing any form of damage to the super villains. It could be assumed that large scale wars have taken place in the Dragon World, much like they have in the real world. However, since the Dragon World is united into a single, global monarchy, it is unknown what purpose the military serves when villains are not terrorizing the Earth that police cannot handle. It may be possible that King Piccolo's original appearance some 300 years before the events of Dragon Ball encouraged the then-king to create a military for just such purposes.
When compared to other races of Dragon Ball, average humans are significantly weak. It is said that a single Saiyan infant would be capable of destroying the average human population on its own (in fact, Goku was sent as an infant to Earth with that exact purpose). The only "normal" human whose power level was reported was a farmer, who Raditz said had a power level of 5. Though his level is probably representative of normal humans, there exist some particularly powerful humans, such as Master Roshi, Tien Shinhan, Krillin, Yamcha, Chiaotzu, Mercenary Tao, Master Shen, Yajirobe, Grandpa Gohan, Olibu, Uub, and Hercule. Many of these human fighters are also Z Fighters. These few humans have learned to use ki and are among the strongest beings in the universe. Even with such abilities, this select group of humans are still considerably weaker than many aliens with the same ki-manipulation abilities. As stated in the manga, it is possible for any human to use their ki and gain powers like the Z Fighters. For example, Gohan easily teaches Videl how to use ki to fly.
Mr. Satan is considered to be the world's strongest man by humans. In reality, he pales in comparison to humans like Krillin and Yamcha, who possess a lot more power. However, this is mostly due to the fact that these humans were granted more opportunities to increase their strength; for example, Krillin had his potential unlocked artificially by Guru and was trained by Master Roshi, and Yamcha trained under King Kai. All of the human Z Fighters climbed Korin Tower and trained under Kami, all things that Mr. Satan never had the chance to do.
It is also worth note that there are many strong humans who are also considered to be non-powerful. These include Nam, King Chappa, and Bora.
It has been shown that humans and Saiyans are able to produce fertile offspring (such as Gohan), but no explanation on the biology that permits this has been given.