21mm wrench and socket. Do not twist the bolt, only the nut. The bolt has two grooves that engage in ridges on the inside of the torque struts shown in the OP's first picture. The control arm is the part that the bottom of the shock and the sway bar link attach to and supports the weight of the car. The torque strut prevents fore and aft movement of the control arm and is used to adjust the caster via the special bolt.
Mark the orientation of the bolt head and try to determine which way the torque strut is offset relative to the bolt so you can maintain the original alignment during reassembly. Also, measure the height of the ball joint end of the original torque strut before you loosen the nut and bolt. When you reinstall, match the height and tighten the nut. This will approximate the orientation of the strut when the car is on the ground so you don't preload the rubber bushing. As I recall, the torque is 80NM then 120 degrees of additional rotation for the nut. The ball joint is torqued to 50NM then 60 degrees.
Edit 9-12-12: The measurements I mentioned above are performed without the strut attached to the steering knuckle. Only the bushing end installed and the ball joint end free of any attachments. M