Custom Property Sets are customizable sets of information that can be defined and attached to folders, documents and users. These sets of information are used to further define the document, folder or user.

A common use for this feature is to allow documents to be searched based on custom information, rather than just their name or content.
Custom Properties can be defined globally or at the domain level. This allows you to create a custom propery set that only appears in one or selected domains. This way, Custom Property Sets that do not apply to certain domains can be hidden from view.
Examples and usage scenarios
How to create Custom Property Sets