The rover's "arm"

The Robot Arm holds and maneuvers the instruments that help scientists get up-close and personal with martian rocks and soil.

Much like a human arm, the robotic arm has flexibility through three joints: the rover's shoulder, elbow, and wrist. The arm enables a tool belt of scientists' instruments to extend, bend, and angle precisely against a rock to work as a human geologist would: grinding away layers, taking microscopic images, and analyzing the elemental composition of the rocks and soil.
At the end of the arm is a turret, shaped like a cross. This turret, a hand-like structure, holds various tools that can spin through a 350-degree turning range.

At the tip of the arm is the turret structure on which 5 devices are mounted. Two of these devices are in-situ or contact instruments known as the Alpha Particle X-ray Spectrometer (APXS) and the Mars Hand Lens Imager (MAHLI). The remaining three devices are associated with sample acquisition and sample preparation functions.