Akuma are mortals or Exalts who are warped by their devotion to a Yozi. In mortals, this may result in their corruption into a Demon-Blooded, and mortals and Demon-Blooded alike may be twisted further via demonic mutations.
The Infernal Exalted are often associated with the akuma, as most akuma are Infernals (even if they weren't yet Infernals at the time of their Exaltation) and likewise, most Infernals are akuma. That said, "Infernal" and "akuma" are not synonyms, though casual use can sometimes conflate or confuse the two when discussing Exalted akuma. The select few Infernal Exalted who are not akuma are known as the Green Sun Princes, and they hold positions of great prestige (and great envy) in the courts of the yozis.