To educate those who want to know, here is a copy/paste:
"[font=Verdana, Arial, Helvetica, sans-serif]Spur gears have straight teeth and are used to contact parallel shafts. Spur gears have a high pitch whinning noise associated with them similar to the noise reverse gear generates.[/font] [font=Verdana, Arial, Helvetica, sans-serif]Helical gears are used in standard factory transmissions. The teeth on Helical Gears lie along a helix, the angle of the helix being the angle between the helix and the pitch cylinder element parallel with the gear shaft. Helical gears can connect either parallel or nonparallel nonintersecting shafts. Such gears are stronger and quieter than spur gears because the contact between mating teeth increases more gradually and more teeth are in contact at a given time. [/font]
[font=Verdana, Arial, Helvetica, sans-serif]Helical gears have one disadvantage when compared to spur gears. When they are loaded a side thrust is created that must be absorbed in the bearings."[/font]