What are “extensions” (in Radiant)?

Part of what makes RadiantCMS great, in my opinion, is that it fulfills the need for most content-management scenarios with pages, layouts and snippets. It’s flexible and puts minimal restriction on the structure of the output. It reaches the 80% window. However there are those cases in the missing 20% where you might have other needs, for example:

* Integration with some back-end data service
* Management of a complex data structure that doesn’t fit into the Radiant page-layout-snippet structure (or fits poorly)
* Receiving and processing input from the website visitor through forms or other means
* Complex manipulation of content information that would be impossible or too cumbersome using Radius

In 0.5.2 and before, page “behaviors” fulfilled many of these concerns. For example, the Mailer behavior processes form-to-email submissions, and the RSS Behavior inserts the headlines from a news feed into a page. However, “behaviors” are focused on processing individual pages and are not well suited to, say, manipulating other models in an administration interface. This is where “extensions” come in.