A Berserker is a powerful type of combat cyborg first developed and employed by the United States Colonial Marine Corps specifically for Xenomorph-clearing operations. Each Berserker unit consists of a heavily armed and armored combat exosuit encasing a human core, which is sealed inside and hard-wired into the suit's control systems via a neurological control system. During missions, the human operator is fed with a potent cocktail of stimulant drugs that renders them hyper-aggressive and essentially unstoppable.
Initial Berserker models, developed by the USCM, were known as the Mobile Offensive ExoWarrior (MOX). Later versions, assembled by the Weyland-Yutani Corporation and operated as part of company-run hunter/killer teams, were known by the similar name Mobile Assault Exo-Warrior (MAX).
Berserkers were very heavily armed, capable of eliminating entire swarms of Xenomorphs single-handedly, and were equipped with sufficient acid-resistant armor to protect them from the creatures' corrosive blood. Typical armament for Berserker units include pulse rifles of various calibers, grenade launchers, and flamethrowers, all of which were integrated into the suit's arms. Later MAX suits weighed upwards of 10 metric tons.
Control of the Berserker was effected through a combination of a neurological interface, using probes drilled into the operator's brain, and the manipulation of physical controls within the suit's arms and legs. The operator was assisted by an advanced computer program that largely automated many of the Berserker's actions, such as aiming the various weapons and tracking targets.
The human controller at a Berserker's core was sealed inside and the suit was never opened under normal conditions — indeed, Weyland-Yutani specifically forbid their hunter/killer teams from accessing the machine's interior. When not in use, Berserker operators were kept in a medically-induced coma, such was their destructive power. When activated, or "fired", the operator was then fed with an incredibly potent cocktail of powerful stimulant drugs that drove them into a blind frenzy, at which point they would simply be unleashed upon the Xenomorphs against which they were targeted. Their destructive rampage was often essentially unstoppable, and could even prove to be as much of a threat to friendly units as the enemy.
Officially, Berserker operators were volunteers. However, at least in the case of the units assembled and run by Weyland-Yutani, they were in fact unwilling participants sealed inside against their will and kept in check through the mixture of drugs used to control them. Operating the suit was incredibly taxing both physically and mentally, and it was not uncommon for operators to die as a result of repeated deployment.
While it was possible for an outside human to enter and operate a suit, even with no training whatsoever, the experience was incredibly painful (owing the the suit's neurological control system, which required controls to be bored into the brain) and massively taxing mentally.