Today, PaperDemon art submissions have both galleries and tags systems.
Galleries are categories that are hierarchical and are created and maintained by site admins. Users cannot add new galleries. This keeps things organized and prevents duplicate galleries from being created
Tags are much more flexible and can be created at any time by members of the community. However they can lead to disorganization and fragmentation due to users unintentionally using the non-canonical tags.
Both serve the same purpose of allowing us to organize and label submissions. For this reason, we're working on consolidating on tags in 2020. Read the Tags proposal doc to learn more about our plans to consolidate and migrate galleries to tags and let us know if you have any feedback.