The publisher is an extension component running in the content repository server.
Its original goal was to retrieve in one remote call the information you need to display on a page. The result is returned as an XML document.
However, the information that can be requested from the publisher consists of more than just XML dumps of repository entities. The publisher provides all sorts of extra functionality, such as 'prepared documents' for publishing with support for document-dependent content aggregation, or performing diffs between document versions.
The publisher was developed to support the needs of the Daisy Wiki, but is useful for other applications as well. Suggestions for features (or patches) are of course welcomed.