Alongside Jobs, Races determines a character's usable abilities, stats, and equipment in Paradox.
Every race comes with a list of armors that can be equipped, skills that are enabled for use, and abilities for passive benefits. Races also give additional skills and abilities by leveling them up, through Job EXP via battling or consuming seeds. A Race is considered "mastered" at level 10 and cannot level further but most races unlock more advanced versions once mastered.
Races can be freely changed by the priest at the Ilias Temple.
Some races require a Race Change Item before it can be selected which are found throughout various locations in the world.
Basic Races are the weakest form available to characters.
Intermediate Races are when you start choosing what sort of role you want your character to play.
Advanced Races are the strongest forms your companions will take.