The Silicates were created to be servants and soldiers; however, they developed an intelligence and sentience of their own, after they were infected by the Take a Chance computer virus created by Dr. Ken Stranahan (name from the show's visual effects supervisor). This sparked an AI rebellion by the Silicates, who attempted to free themselves from human rule. This long war continued for many years, until the Silicates captured military space craft and escaped into space. As they went into space, the Silicates suffered from a lack of maintenance which caused problems for them.
The remnants of the Silicates that fled into deep space serve as mercenaries and actually aid the alien Chigs in their war against humanity.
The AIs (Artificial Intelligence) were manufactured by humanity to serve them and, as such, appear as humans but with enough differences to appear as machine creations, namely the rifle sight-like crosshairs in place of pupils. They were made to be beautiful and physically appealing by their makers. The surviving Silicates that fled into deep space have been suffering from a lack of adequate maintenance for many years, and thus frequently possess minor damage to their outer covering which reveals their machine parts underneath.
Silicates were primarily designed to be domestic servants, or even pleasure slaves, and not particularly for hard labor (which would be done by heavy machinery). As a result standard Silicates are actually not that much stronger than a human, and because they were not originally designed to be used in combat, it is not particularly difficult for a trained human soldier to defeat them in hand-to-hand combat. This is partially offset by the fact that Silicates are not hindered by physical pain, and cannot experience the emotion of fear.
Silicates communicate with one another through modulation schemes made by wireless telephone which comes across to humans as a series of electronic beeps and chirps. This wireless network allows each AI to know the position and operating status of the other units. Their mechanical nature allows them to store information and retrieve it making them excellent in information gathering which can be shared with their colleagues when demanded.
As the AI Silicates were created as a "servitor" species, they were programmed to understand abstraction, but with programming that restricted original  thought and creativity, which leaves them to simply imitate rather than create. Had the "Take a Chance" computer virus not been created, it is likely the Silicates would have remained within their servant race role. As such, risk-taking has become the prime ideology of the AI Silicates which results in them seeing activities as a risk or gamble. The first "risk" was the indiscriminate killing of their human creators in the AI War, which lasted for ten years. The Silicate robots refer to humans as "carbonites", because they are carbon-based life forms.
Because the Silicates were programmed to comprehend abstract thought, but also restricted from formulating original thoughts, and do not normally possess emotions, such as fear, the Silicates are capable of understanding that humans experience fear, albeit this comprehension is on an academic level. This made the Silicates a deadly enemy in the AI Wars, because while they experienced no fear themselves in combat, they realized the value of random and savage attacks meant to terrify and demoralize humans. While the Silicates were incapable of originating such tactics on their own, they simply needed to imitate the long history of terror tactics used by human armies.
A Silicate's inability to experience emotion, as mentioned above, is contradicted by two episodes: 1x18, titled 'Pearly', in which a Silicate displays concern for the welfare of and affection for a Silicate that is badly injured, and 1x03, titled 'The Dark Side of the Sun', where revenge upon the protagonists for the death of another Silicate is attempted. The emotional capability of Silicates is never explored by the series, so it is unknown if these displays of emotion were out of character, or the intentional development of character types.
It is strongly implied that the AI Wars were not a primarily "conventional" war, with each side gaining and losing territory, but largely consisted of Silicates infiltrating human societies and committing random acts of terrorism and sabotage. Fighting was not limited to "front lines" as the Silicates intentionally attacked places humans thought they would be safe in order to terrorize them: Shane Vansen's parents were killed when a group of Silicates drove into her middle-class suburban neighborhood, randomly storming her house. The Silicate's gambling-centered ideology even extended to combat tactics: they randomly chose to attack Vansen's home as the result of a coin toss.
The tide of the Human-Chig war began to turn after initial Chig successes because Chig battle-tactics favor large-scale and direct military assaults: the human military switched to asymmetric guerrilla warfare, which the Chigs were not conceptually experienced with fighting. The subsequent alliance between the Chigs and the remnants of the Silicates, who are quite experienced at non-conventional warfare and terror tactics, partially made up for this deficit in Chig strategy.
In other works
The Synthetics from the Alien movies are similar in appearance and role; however, they served as assistants.