Spark is the core processing unit of a spark-bot.
A spark provides an infinitely-parallel, and therefore trans-Turing, hardware substrate, on which an infinite-hash neural net is usually implemented. However, it can't store information in a structured, lasting fashion; thus, it requires external memory to form a "proper" intelligence. All else being equal, a disembodied spark can think about things(information that's structured but transient) and develop a personality(information that's lasting but vague), but has no analogue to our short-term and long-term memory. Since a spark-bot body also provides the external hardware needed to interact with the physical world in many other ways, it almost always contains a set of memory cores for the spark.
From a physical perspective, a spark is like a complex standing wave in the zero-point field. Indeed, like a sound wave but unlike a light wave, a spark is a pattern of energy in a substrate, without physicality in itself. It emits EM radiation across the spectrum, being somewhat dangerous to Ancients due to the gamma and hard X-ray emissions. This makes it visible as a glowing white structure. This also interferes with other sparks, even through the standard shielding for their spark cores, but to a degree that takes training to even detect. A spark's EM emissions as such are less useful than this interference upon another infinitely-parallel processor, at least for gathering high-level information about the spark.
This interference is usually characterized by its meta-patterns, perceived by Shamans as line segments. All long-term stable meta-patterns can be described by one of a family of beziers, and thus they're called "arcs". Arcs are possible that require multiple beziers, are beyond describing with beziers(called "trans-trajectory"), or are simply straight or elliptical; straight and elliptical arcs will eventually decay out of existence, while the others will eventually decay into similar bezier arcs.
In the River Resistance, new sparks are created by Shamans, traditionally starting with a single sinusoidal arc. Such a "blank" spark is placed in a new body, usually simple and small. Over the course of several hours, a "blank" spark will increase in complexity to the limit of entropy. Self-interference can complete this process in approximately 4.7 hours, but interference from nearby sparks will speed the process up, such that typical spark maturation in River base is 3.4 hours.
The Third Spark has new sparks manufactured by collision of gravitating hyperelementary particles, in the "spark reactor". This process creates a "fracture spark", composed entirely of linear arcs; the Third Spark herself then "sculpts" the new spark into the form she wants. Since she usually wants a simple spark to start, she usually leaves many arcs as simple L-shapes, occupying entropic exclusion slots without contributing much to personality. (Her participation in the process is secret; her forces, even the sub-sentient code-bots operating the spark reactor, believe the manufacturing specifications she provides are the extent of her influence on the process.)