The forearms contain many muscles that help with grip and rotation of the lower arm. Exercises such as wrist curls target the muscles directly, while heavy deadlifts and farmers carries can build them indirectly.
The Yes4All Wrist and Forearm Blaster offers another effective method of working the forearm muscles directly. Simply attach a weight to the cable and roll the bar forward and backward to lower the weight up and down to really feel the burn.