Attribute groups are collections of custom attributes which extend only a subset of products, either via inheritance (category groups), by being explicitly added to individual products (property groups), or by being shared across multiple attribute groups (reference groups).
For guidance on which type fits your data, and why category groups are recommended over property groups for performance at scale, see Product attributes.
Reference groups
Create reference groups for custom attributes shared across multiple attribute groups
Category groups
Create category groups of custom attributes inherited by subsets of products
Property groups
Create property groups of custom attributes added ad-hoc to individual products