DITA is a philosophy for topic-oriented information management along with structured document data definitions for delivering it. It isn’t a general-purpose reusable technology like DocBook. Instead it focuses exclusively on creating a document model that best serves the diverse needs of your readers across different and equally valid scenarios.
A lumpier way to put it is that DocBook is to DITA as a chicken egg is to an omelet. You can’t unscramble an omelet, and when you figure out how, you shouldn’t! You can use DocBook to provide a DITA-like approach but you can’t use DITA to provide DocBook’s functionality.
How do DITA and DocBook compare? They don’t is how.
DocBook is an open-ended technology for designing anything. DITA is a focused technology for delivering a learning model.
An open standard created and maintained by self-sacrificing volunteers and BIG companies it is beloved, high-quality, and suffers the same ills and pangs of all human endeavors