What's the difference between a Facebook Product Catalog and a Facebook Product Set?

A set, or Product Set in Facebook, is a group of items in your catalog that you can feature in ads.

A Catalog is a container in which the products or item information is held.

You can find all of your Product Sets in Facebook Commerce Manager via the Catalog tab under Sets.

For more info on best practices for managing Facebook Products Sets, please refer to Facebook's best practice guide for managing sets in a Catalog.