Gear box:
Connects the low-speed shaft to the high-speed shaft and increases the rotational speeds from about 30-60 rotations per minute (rpm), to about 1,000-1,800 rpm; this is the rotational speed required by most generators to produce electricity. The gear box is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gear boxes.
Generator:
Produces 60-cycle AC electricity; it is usually an off-the-shelf induction generator.
High-speed shaft:
Drives the generator.
Low-speed shaft:
Turns the low-speed shaft at about 30-60 rpm.