Add-ons are composed of several types of elements.
Executable Element - When the add-on is executing, this code is run. These elements include active scripting like vbScript, inline scripts like PHP, referenced to Visual Studio dotnet classes and much more.
Content Elements - When the add-on is placed on a page or template, these elements are placed on the page.
The Form Element - An xml structure used to create simple settings pages. For instance, if your code uses a site property to display the administrator's name, you could save the name in a site property and use another add-on as a setting page that the site owners could easily access and modify the site property.