The product group object contains information about a product group record.
A product group is a way of categorizing products. You can create and maintain product groups in System Setup > Product Groups.
Opportunity groups
Take care not to confuse product groups with opportunity groups. Opportunity groups categorize opportunity items.
When you add products to an opportunity, opportunity groups are created with the same names as your product groups. You can change and rename opportunity groups, nest groups, and even delete them.
Though the names may be the same, opportunity groups will not return product group objects. They are opportunity items and return opportunity item objects.
Objects that return product group objects
product_group is an object of the product object, so you may access it anywhere you may access the product object.