Outwardly, Expanoids are a race of cyborgs virtually indistinguishable from common Simulants. However, unlike most Simulants who were built for war and are genocidal, the Expanoids instead desired to enslave humanity rather than simply making them extinct.
According to the mechanoid Kryten, they were named Expanoids since their transistors had the capacity to double in power every two years. Kryten also claimed they were cold-blooded psychopaths, with no qualms about using hackneyed old cliches to achieve their ends.
According to to the Expanoid spokesman/leader, 4 of 27, they were sent from a distant future where humans had themselves realised that they were not mature enough to handle advanced technology. 4 of 27 had been programmed to alter the timeline so that humans would never create advanced technology, and therefore never become a great danger to themselves. Therefore, 4 of 27 saw himself as a "good guy", and for whom the ends always justified the means.