The race of Humans live on planet Earth. They are the median between the three classes and can play all three of their class roles average, but with no specific focus on any one benefit. Therefore it is impossible for Humans to be the best at anything. They also have Saiyan blood.
The martial artist is the melee class for humans. The most known members of the Martial Artist Class are none other than the infamous Saiyans and their off-spring. It should be noted that physical attacks serve a better purpose to this class rather than Spiritual moves.
Very little is known about the class of the engineer at this time as they were not playable during the various beta phases. However it is assumed that they will utilize technology and serve primarily as craftspeople. Notable Human Engineers are Bulma.
Individuals who invest their power in the spiritual energy of the universe. Their attacks revolve around a Ki energy source and a signifigant number of them are ranged. It is speculated that these few will serve as support roles in DBO. Notable Human Spiritualists are Tien and Krillin
It is notable that each class has advanced branches which can be met after reaching certain milestones within your characters progression.