Q: What's cross-site publishing in SharePoint Server 2013, and when would I need to use it?
A: Cross-site publishing is a new publishing feature that lets you create and maintain content in one or more authoring site collections. You can also publish content in one or more publishing site collections through Search Web Parts. It's particularly useful for when you need to store and maintain content in one or more authoring site collections, where you need to display content in one or more publishing site collections.
According to Microsoft, the cross-publishing feature includes several different benefits, such as the ability to separate content authoring from branding and rendering. It can also be used across different site collections, web apps, and farms, as well as permitting a mix of authored pages and list content.
SharePoint admins will be happy to know that the new feature complements the existing publishing method, where you can use a single site collection to author content and make it available to your site's users.
See the TechNet article for more information on the new cross-site publishing capability.