The Cybertronians is the true name of the "species" that humans call "the Transformers". Cybertron is their homeworld, an artificial planetoid orbiting a distant star. All factions of Transformers are Cybertronians, the Autobots, the Decepticons, the Maximals, the Predacons, even the Vehicons. Not all Cybertronians can transform however, the "art" of transformation was not developed until the Second Cybertronian Civil War, the first war fought between the Autobots and the Decepticons. However, since that time, all Cybertronians can transform, only those predating the Second War may not have this ability.
Cybertronians are a machine race, they do not breathe, consume food or drink water like biological races. Instead, Cybertronians use a rare substance called Energon as a fuel/life force. In it's raw form it is actually harmful to them (although not to organic life) but in its processed form, it is essential to their survival. Cybertronian technology has developed to the point where they can convert most forms of energy into Energon.
All Cybertronians carry a "spark" inside them, as long as this spark endures/still intact, the Cybertronian can be rebuilt, no matter how much damage the unit has sustained. If it is extinguished/destroyed, the Cybertronian cannot be rebuilt and is completely dead. It is a common belief amongst the Maximals that their sparks are a part of or a fragment of Primus, the mythical creator of the Cybertronian race.
In the so-called Generation 2 comic books, "Cybertronian" was a name of a faction of Decepticon descendants that took to calling themselves that in order to distinguish themselves from the other Decepticons and the Autobots. They're sometimes more correctly referred to as Second Generation Decepticons or some variation therein.
- These Cybertronians were a result of Decepticon rediscovery of Bio-Morphic Reproduction ("budding") ~4 million years ago on Cybertron. The Decepticons were able to rapidly expand their population using this method.
These new Decepticons left Cybertron in the hands of petty warlords unfit for the future empire and took to the stars to conquer other worlds.
In the modern era, the Cybertronian Empire has adopted an aggressive expansionist stance. They claim to have advanced their society beyond the need for war, but hold little regard for non-Cybertronian lifeforms. Under the cause of improving the quality of life of their citizens, they have conquered many worlds, enslaving or exterminating the native population and cyberforming the planets to create "little Cybertrons."
This was apparently under the direction of the Decepticon progenitor known as the Liege Maximo, who rules them from the planet-spanning structure known as the Hub, which harnesses the networked power of the cyberformed planets. The construction of this network was later revealed to be part of a gambit by the Liege Maximo to ascend to the plane of the Dark Gods.