There are three kinds of extensions, that can add many kinds of functionality to your default CMS Made Simple install. They are called tags, user defined tags, and modules.

Tags

Tags are the simplest form of extensions. They are designed to accomplish just one small and specific task.

There are a number of custom tags available with CMS Made Simple. To find what kind of tags are available look in Extensions » Tags in the Admin Panel.

To insert any of these in a template or a page, simply type e.g. {content}. Many of these Smarty tags are used as placeholders in a template, i.e. placeholders for content, navigation, breadcrumbs etc.

Website developers who have a bit of PHP experience will find it easy to create and share their own custom tags.

User defined tags

Users can also create their own tags to insert in templates or pages., these are called user defined tags. They are snippets of php code (but without the <?php and ?> surrounding them), providing the ability to add re-usable pieces of php functionality to your site. User defined tags are inserted in templates and pages just like tags: {tagname}.

Typically, user defined tags provide a utility that is special to a website, and likely won't need to be re-used on another site. Also they are typically small and used for simple tasks.

Share your user defined tags and use those that others have created(external link).

Modules

Modules are the highest level of plugin in the CMS Made Simple environment. They are designed to allow developers to implement complex tasks within CMSMS. A module typically provides advanced functionality, usually interacts with the database in complex ways, and may provide numerous reports or forms on the website. Additionally, a module may have an administrative interface to allow manipulating its data and its settings.

An extremely well defined API (Application Programming Interface) has been written to allow module developers to write complex, intricate, and fully functioning applications for use within a CMSMS powered website.

There are a few modules included with the default installation of CMS Made Simple. Other popular modules are Frontend Users, Album, Calendar, Guestbook and Form Builder.

The ModuleManager module (included with CMS Made Simple) allows browsing a list of available modules, reading about them, and then installing them on your website.

To insert modules in a template or a page, you actually use the module name as a parameter to the {cms_module} tag. It looks like this: {cms_module module='modulename' parameter1='this' parameter2=5 parameter3='that'}. It is normal for modules to accept parameters to effect changes to their default behavior, though it is not always required.

Read more

You can read more about extensions in the CMSMS documentation(external link).