Keywords are an MDItem that are frequently associated with PDFs or image files. Keywords can be set or viewed in DEVONthink Tools > Show Properties and there is a column option available for “Keywords” that can be added to the columns displayed in list views, three pane view, etc. DEVONthink provides a script in Scripts > Tags > Convert Keywords to Tags (an optional add-in in the Support Assistant). See this, this, and this. Not an exhaustive set of references – but might help with some background.
From an end-user perspective – there’s really very little difference between keywords and tags, just a difference in the implementation which has more to do with different features evolving at different times than anything.