To build your forearms and grip strength, you need to train all three grip functions: support, crushing, and pinching strength. This exercise will take care of the last two categories.
These build significant functional strength and hypertrophy throughout the entire musculature of the forearms, not to mention Herculean grip strength. Note the pause at the top and bottom of each rep.
Other exercises include: fat grip lifts, plate pinching drills, towel pull-ups, rope climbing, rope pulling, and hex dumbbell pinching exercises. (More info here: Freaky Forearm Training.)