An Aragami is a colony of hundreds of thousands of cells uniting under a cell core. These cells, called "Oracle Cells", are the only type of cell in an Aragami forming flesh, blood and bone all the same. Oracle cells have a devouring nature which caused them to spread and devour everything in their path. However, Oracle cells do not evolve like normal cells do; in fact, each Oracle cell in existence is exactly the same as it was before the outbreak.
Each Aragami has a "core", from which all the Oracle cells in the Aragami are controlled. The core is the command center over the entire colony while the individual groups of cells form working parts of the entity such as an eye, fang, or tail. The only way to truly kill an Aragami is to extract the core, thus making all the normal cells lose their cohesion. The only weapon capable of doing this (whilst still being portable) is the God Arc, which removes it by "devouring" the Aragami.
Generally violent in nature, Aragami crave only to consume. Indeed, this is how they adapt and evolve to fit new surroundings; they will devour native scenery and Aragami in the area until the Aragami's cells adapt to the conditions. Notably, the Aragami seem to be starting to evolve in new, unsettling ways: some of the newer Aragami are starting to resemble humans.