In eZ Publish, an extension is like a plugin through which you can add new functionality to the CMS without changing the standard software kernel. The eZ Publish extension system is very powerful, as it allows the CMS to be extended in different ways, such as adding new features, or changing the standard behavior of the system's basic functionality.
With this system, you can keep the CMS constantly upgraded to the latest version without the fear of losing your code. In fact, in most cases, the extension will continue to run and in the worst case, you should also upgrade the extension to make it compatible with the new version.
It's possible to create different kind of extensions, such as:
Operator: This extends the standard template operators. (We will talk about them in detail in the next chapter.) For example, we can make new operator that can work with a string, or that can connect to external web services to perform some routine such as downloading...