They resemble blue stars with white circuits lines, while some are Prismatic.
Technically they are engineered plasma life forms, like Sparks and Embers they give life to their hosts, and sentience. However, their bodies are not animated by their Plasma Hearts.
Within each Heart is the Life Code which decides their characteristics. Certain Hearts are given modified code, to create mechs to serve a certain function. Those with naturally occuring codes are called Natural, while those with manufactured/designed Life Codes, are called Coded.
Each Heart is born when the code is transmitted into an energized field of Engen. The higher the quality, the better the chance of a Heart igniting into existence. However, this code operates by using Maxion Prime's Ember, to jump start these new Hearts. Thus when he escaped his prison in Chromium's core, it became increasingly harder for new Hearts to be born. When he was revived as Maxion Prime, it became impossible as he consciously denied each new Heart his life force.
Among the Coded, are Super Plasma Hearts, Plasma Hearts given codes, to attempt to replicate the power of Maxion's Life Code disguised Ember. They are Prismatic in appearance, while powerful compared to normal Hearts, pale in terms of power when it comes to an Ember.