The Krotons evolved from predatory quasi-organic tellurium based crystal which could mimic their prey's abilities. At some point in their history they encountered a Human servo robot and, mimicked it, thus attained semi-sapience. As they evolved they became semi-sentient armoured crystalline beings which fed on/psychologically linked to mental vibrations.
The Kroton Absolute were overseen by the First Lattice, originally a fortress on the Kroton's home world, it contains the four highest brains in the Kroton Absolute. The First Lattice was moved from the Kroton home world when the war began with the Metatraxi, it took over four thousand Krotons to lift the First Lattice into orbit and it had exhausted all of them. The Krotons had crystalline bodies which can be broken down and reformed to suit the environment and the situation which they are in.
The Krotons encountered the Doctor on at least two occasions. On one occasion the Doctor killed a pair of Krotons stuck on a planet of Near-Human slaves known as Gonds. The Krotons had enslaved and manipulated the lives of the Gonds for generations so that they might power a dynatrope (a Kroton spacecraft) with mental energy and escape the planet. The Doctor assisted in the Krotons' destruction with sulfuric acid.
In his sixth incarnation, the Doctor found a dynatrope with at least four Krotons on board and assisted a group of Humans in destroying two of them. He did allow the two survivors to escape.