Akuma are Exalts who are warped by their devotion to a Yozi. Only the Exalted can be turned into Akuma, and any type of Exalt can be turned into one.
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.
Although an Akuma is vastly more powerful than a normal Exalted, they have the fatal flaw that they have no free will whatsoever as the Desecration that turned them into one also rewrote their very beings into something more useful to their new bosses, turning them into a complete puppet of their Yozi master and warping them into beings with similar personalities.
There is no way to restore an Akuma back into his/her former self while they live. If an Akuma is killed, their Exaltation will remain tainted by the Yozis' power, and will take several lifetimes to completely purify it, and during that time the affected Exalt will have to avoid contact with any kind of demon, which is not easy as the demonic taint within the Exaltation will try to attract the former Akuma's reincarnation towards demonic energies and influences.
The first Akuma was Thrice-Damned Gorol, a Night Caste Solar of the First Age. The last known survivor of the Ancient Lintha, Lintha Ng Hut Dukantha, is also an Akuma. He's also the strongest living Akuma in the Age of Sorrows. In Return of the Scarlet Empress, it's also revealed she's an Akuma too.