(This is a work in progress.)
Facets are markers on document types that instruct Nuxeo (and any part of the system that cares about them) to behave differently, or that are automatically set on some document instances. The following facets are currently in use by Nuxeo, but others could be defined by code using Nuxeo.
Folderish
This should be set on types that are to be considered "folders", and therefore for which a list of children may be displayed in the navigation tree and other user interface areas.
HiddenInNavigation
This should be set on types which you don't want to see in regular listings of documents and folders.
Orderable
This should be set on folderish types for which maintaining the order of children is important. (CoreSession.orderBefore can be used only on documents contained in orderable folders.)
Versionable
This should be set on document types for which you want to have versioning.
Publishable
This should be set on document types for which you want to have publishing.
Immutable
This is automatically set by the system on document instances which correspond to immutable objects (archived versions and proxies).