Destroyed Mechs are entirely mechanical inside, so there is no "pilot", implying they are unmanned drones.Ones who control mechs can put them into guard/patrol mode, give special orders violating their usual protocol and assign priority targets. These commands are followed even if the commander is dead implying that they have primitive A.I. that allows them to some extent think on their own. It seems that harnessed children can also order Mechs such as when Karen Nadler ordered a Mech to execute Boon.
They can shoot bullets and rockets from their arms and have a multi-laser targeting system they use to triangulate their enemies. The lasers are blue while targeting but once locked on they turn orange. Additionally they can fire an energy beam to stun humans, especially children in order to harness them. The Mechs possess two arms, one manipulator-arm with a clawed hand (capable of tearing into a school bus), and another "arm" that is really a weapons-stub for a built-in gun. Mechs walk on two, digitigrade legs - as opposed to the six legs of the Skitters, which was the first hint that the Skitters did not design them.
Their armor is resistant to small arms weapons, and they seem to respond to motion and sound. Despite their extremely durable exterior however they are not invulnerable and can be destroyed with a large enough explosive force, such as RPGs, C-4, or well-placed hand grenades. The 2nd Mass disabled one and Pope has taken the job of finding how they work. Examination of a Mech's turret revealed that the rounds used are in fact made from salvaged military ammunition, with bullets cast from Mech metal (vaguely similar to human depleted uranium-tipped rounds, but much more powerful). These re-purposed rounds are capable of shooting through a car engine block. The rounds that were recovered were .45 caliber and could be fired from a revolver, easily penetrating a Mech's armor with a single shot, warranting more ammunition to be made from the remains of previous Mech in their possession.