From Dragon Ball Encyclopedia
Humans (人間, Ningen) are Earthlings.
With the exception of the uncommon hair colors and occasionally odd features (ranging from longer lifespans to lycanthropy), Humans are mostly identical to their real world counterparts. They 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 were the main inhabitants of Planet Earth. They shared the planet with two other types of Earthlings: monsters and animals. In fact, they were led by an anthropomorphic Dog, King Furry. Humans were part of a large military that mostly consisted of ground troops, tanks, fighter jets, and large naval ships. Earth's Military usually appeared when attempting to defeat many of the villains who caused massive damage to Earth (examples being Future Cell and Majin Boo), but were always easily wiped out without causing any form of damage to the villains. It could be assumed that large scale wars had taken place on Earth, much like they had in the real world. Since Earth was now united into a single, global monarchy, it is unknown what purpose the military served – when villains were not terrorizing Earth – when police and Town Militia also existed. It may be possible that Piccolo Daimao'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 the other races in Dragon Ball, average Humans were significantly weak. It was said that a single Saiyan infant could be capable of destroying the average Human population on their own (in fact, Son Goku was sent as an infant to Earth with that exact purpose). The only "average" Human whose battle power was reported was a Farmer, who Raditz said had a batlle power of 5. Though his level was probably representative of normal Humans, there existed some particularly powerful Humans, such as Turtle Hermit, Tenshinhan, Olibu, Kuririn, Chaozu, Yamucha, Tao Pai Pai, Crane Hermit, Yajirobe, and Oob. Most of these Human fighters were part of the Z Warriors. These few Humans had learned to use Ki and were among the strongest beings in the universe. Even with such abilities, this select group of Humans were still considerably weaker than many aliens with the same Ki-manipulation abilities. As stated in the manga, it was possible for any Human to use their Ki and gain powers like the Z Warriors. For example, Son Gohan easily taught Son Videl how to use Ki to fly.
Mister Satan was considered by the masses to be the strongest Human, but in reality paled in comparison to Humans like Kuririn and Yamucha, who possessed much more power. This was mostly due to the fact that these Humans were granted more opportunities to increase their strength. For example, Kuririn had his potential unlocked artificially by Grand Elder and was trained by Turtle Hermit and Yamucha was trained under North Kaio. All of the Human Z Warriors had climbed Karin Tower and trained under Kami, all things that Satan never had the chance to do.
It had been shown that Humans and Saiyans were capable of producing fertile hybrid offspring (such as Gohan, but an explanation on the biology that permitted this has ever been given.
Dragon Ball Online
After Gohan wrote a book in 800 Age that unveiled the secrets of ki and the martial arts, fighting became incredibly popular amongst the Humans, and thus ushered in a new dawn of Human warriors. Kuririn and Tenshinhan created schools to train Human warriors, and even Son Goten and Trunks Brief started a sword school.
This of course greatly aided the Humans when Miira attacked nearly a millennium later and allowed the Humans to gather a resistance against the villain. Humans were also capable of using the Super Saiyan transformation if they wised upon the Dragon Balls.