Product Categories allow you to group similar memberships, services, and point of sale items together. This allows for better organization during the purchase process. For example, you can have a membership product category called "Individual" and assign all of your membership types for individuals to this category. Another for family, student, and so on. On the services side, a product category called personal training and another for classes to group those service types together. For point of sale, some examples are clothing, beverages, shakes, etc.
Product Categories are managed in Tools>>System Settings>>Field Definitions.
Within this screen, you can add product categories and edit existing product categories. You can also mark categories as inactive.
Memberships: Membership Categories
Services: Service Categories
Point of Sale: Product Categories
Once created, you can assign them for each membership type, service type, and point of sale item. When purchasing a membership, for example, you will see all of the product categories organizing all of your types instead of a full list of all types.