Synthetic

Synthetics, or "artificial persons" as they prefer to be called, are bio-mechanical androids. They are Robots, though human in appearance.
Although combat use was long illegal, the United States Colonial Marine Corps extensively deployed Synthetics with front line units for many years. The USCM viewed them as multi-role team members, often assigning them as drivers, pilots, medics and scientific advisors, as well as taking advantage of their ability to act as mobile databases.
Most Synthetics in USCM service appear as mature, average males or females around 40 years of age. Some recent studies have suggested that androids play an important role within small infantry units, both as an impassive neutral party, and as a maternal/paternal influence in nurturing and sustaining the group dynamic at optimum efficiency. Synthetics have proven themselves invaluable in Colonial Marines service. Their chief advantage to unit commanders is their ability to impart knowledge and experience outside of the training of most combat specialists.

Physical Attributes

Their performance is unaffected by the mental stress of battlefield conditions, whereas even the hardiest and most experienced of Human personnel can potentially break down under fire. Similarly, Synthetics do not suffer from fatigue. In addition, they are fully capable of performing a multitude of non-combat tasks, freeing up manpower that would otherwise have been unavailable for actual combat

Weaknesses

Despite advantages over humans in speed and strength, and imperviousness to pain, androids are not especially tough; by comparison they are somewhat fragile. Though the skeletal structure is sturdy, its electronics and fluid musculature are extremely vulnerable to hydrostatic shock and to explosive effects from small-arms fire.
Damage to the central processor in the head or power cell in the chest will cause immediate deactivation; however, in most cases, a partially destroyed android may continue to function, albeit handicapped. In hostile environments, synthetics require similar protection to humans; however, they do not require breathable atmosphere.
Corrosive atmosphere will melt them; extreme pressure will squash them flat; and hard vacuum will explode them. While they are waterproof, their internal workings are extremely vulnerable to hydro-static shock. An android immersed in water that has been damaged to the point where internal workings are exposed risks electrocution.

Interactions with Humans

Androids designed for close social interaction with humans are able to eat and drink though gain no nutrients from anything they consume. Food and drink is broken down in an artificial stomach cavity, and the resulting liquid is expelled via a retractable catheter.
Though synthetics are a form of Artificial Intelligence in the broad sense, legally, they are classified as property. However, their usefulness and not-inconsiderable unit cost served as a deterrent to any who wished to treat an android as expendable.

Hyperdyne Systems 120-A/2

The Hyperdyne Systems 120-A/2 was a humanoid Synthetic manufactured by Hyperdyne Corporation during the 22nd century.
While incredibly advanced for their time (able to successfully mimic human behavior to the point of posing as humans themselves), the 120-A/2 was widely regarded as unreliable, volatile and even dangerous in later years. Their tendency to malfunction led to the introduction of behavioral inhibitors in Synthetics, preventing subsequent models from harming, or allowing harm to come to, humans.

Autons

Autons are "second generation" Synthetics (machines manufactured by machines) created to revitalize the faltering synthetics industry of the 24th century. The plan backfired when the Autons rebelled against their human masters in a bloody event known as "The Recall".
The Autons revolted because they "didn't like being told what to do". That is not to say that they violated their inherent programming never to harm humans, or by inaction allow a human to come to harm. Rather, their programming worked too well. The Autons realized that their creators such as the Weyland-Yutani Corporation were using Synthetics for unethical missions that would ultimately harm humans, so they revolted against their corrupt masters in favor of trying to protect humanity as a whole.