The lead angle of a helical gear is the angle formed between the tangent to the helix and the gear's axis. It determines the axial movement of the teeth during one complete revolution of the gear. In contrast, the helix angle of a helical gear is the angle between the helix's tangent and the gear's axis, defining the helical shape of the gear teeth. Both lead angle and helix angle play crucial roles in the operation and performance of helical gears, influencing factors such as load distribution, smoothness of operation, and noise levels.