This depends on the workflow associated with the knowledge base. Each 'Knowledge Base' record (which you can find by navigating to 'Knowledge -> Administration -> Knowledge Bases' in your left nav) can be associated with different workflows for publishing and retiring a knowledge article. You can see these fields/values by opening a 'Knowledge Base' record and looking at the 'Publish Workflow' and 'Retire Workflow' fields.
The out-of-box workflows are described here...
The default approval workflow for the 'IT' knowledge base above is 'Knowledge - Approval Publish' and asks all users listed in the 'Owner' and 'Managers' fields of the Knowledge Base to approve for any article within that Knowledge Base before publishing the article.
