It's round, a v-belt runs through its grooved edges, and there's a hole in the center to attach it to the shaft of the motor... Your vast experience in this arena tells you it's a v-groove drive pulley. After removing the damaged product, you locate your lawnmower manual, and (with the use of a high-powered microscope) are able to decipher your Genuine OEM Part Number 

The description next to the part number says "Sheave, 7x3/4"

You see black around the edges of your eyesight... Now you have to question everything! Is the world truly spinning, or is it just you? How have you gone your whole life calling this a drive pulley, if it's really a sheave? 

Take a deep breath, you're not alone in this dilemma. This is, one could argue, the ULTIMATE QUESTION: 

Apart from these belts in general being a bit longer than our rubber v-belts, there is one very apparent difference: They're blue.

What's so special about these belts? They are wrapped in Aramid cord and are designed for use in heavy duty applications. But what is aramid? Why is it so great, and why do aramid cord belts look just like Kevlar®?