Helical gear has a different geometry as compared to a spur gear. In spur gear, the teeth are cut at 90 degrees. In case of helical, they are cut at a particular helix angle and sometimes they appear twisted.
In the helical gear system, since the gear is cut at a helix angle, the contact starts at one point. It then covers the entire gear tooth as both the gears rotate. This is a major differentiating factor as compared to a spur gear.
Helical gear thus has a quiet operation and higher load carrying capacity. Helical gears, also due to the type of contact between them, last longer than spur gears.