Software extension, is a file containing programming that serves to extend the capabilities of or data available to a more basic program. It is a kind of list of commands which are directly included in the program. This term often (mistakenly) coincides with the plug-in. When installing software, you may be instructed to take one or more steps related to installing extensions (or these steps may automatically be done for you).

Add-on is often considered the general term comprising snap-ins, plug-ins, extensions, and theme

Theme (computing)

In computing, a theme is a preset package containing graphical appearance details, used to customize the look and feel of an operating system, widget set or window manager....

s.

The format of extensions can vary from one program to another, with for example:

• XPI format for Mozilla family (the browsers Firefox and SeaMonkey
  SeaMonkey
  SeaMonkey is a free and open source cross-platform Internet suite. It is the continuation of the former Mozilla Application Suite, based on the same source code...
  
   , the email client Thunderbird
  Mozilla Thunderbird
  Mozilla Thunderbird is a free, open source, cross-platform e-mail and news client developed by the Mozilla Foundation. The project strategy is modeled after Mozilla Firefox, a project aimed at creating a web browser...
  
   , but Nvu
  Nvu
  Nvu is a WYSIWYG HTML editor, based on the Composer component of Mozilla Application Suite and Gecko 1.7. It is a common WYSIWYG editor for Linux, Microsoft Windows and Macintosh, intended to be an open source equivalent to proprietary software like Microsoft Expression Web and Adobe Dreamweaver...
  
   and Songbird
  Songbird
  A songbird is a bird belonging to the suborder Passeri of the perching birds . Another name that is sometimes seen as scientific or vernacular name is Oscines, from Latin oscen, "a songbird"...
  
   )
• ZIP
  ZIP (file format)
  Zip is a file format used for data compression and archiving. A zip file contains one or more files that have been compressed, to reduce file size, or stored as is...
  
   format for Opera
  Opera (web browser)
  Opera is a web browser and Internet suite developed by Opera Software with over 200 million users worldwide. The browser handles common Internet-related tasks such as displaying web sites, sending and receiving e-mail messages, managing contacts, chatting on IRC, downloading files via BitTorrent,...
  
   Navigator
• JAR
  JAR (file format)
  In software, JAR is an archive file format typically used to aggregate many Java class files and associated metadata and resources into one file to distribute application software or libraries on the Java platform.JAR files are built on the ZIP file format and have the .jar file extension...
  
   format for Java
  Java (programming language)
  Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
  
   (part of the XPI include JAR files)

Extensions and Plug-ins

Extensions differ slightly from plug-ins. Plug-ins usually have a narrow set of capability. For example, the original impetus behind the development of Mozilla Firefox

Mozilla Firefox

Mozilla Firefox is a free and open source web browser descended from the Mozilla Application Suite and managed by Mozilla Corporation. , Firefox is the second most widely used browser, with approximately 25% of worldwide usage share of web browsers...

 was the pursuit of a small baseline application, leaving exotic or personalized functionality to be implemented by extensions to avoid feature creep. This is in contrast to the "kitchen sink" approach in its predecessors, the Mozilla Application Suite

Mozilla Application Suite

The Mozilla Application Suite is a cross-platform integrated Internet suite. Its development was initiated by Netscape Communications Corporation, before their acquisition by AOL. It is based on the source code of Netscape Communicator...

 and Netscape 6 and 7. Therefore, after integration, extensions can be seen as part of the browser itself, tailored from a set of optional modules.

Firefox also supports plug-ins using NPAPI. When the browser encounters references to content a plug-in specializes in, the data is handed off to be processed by that plug-in. Since there is generally a clear separation between the browser and the plug-in, the results are discrete objects embedded within a webpage. The same distinction between plug-ins and extensions is in use by other web browsers, such as Microsoft Internet Explorer, where a typical extension might be a new toolbar

Toolbar

In a graphical user interface, on a computer monitor, a toolbar is a GUI widget on which on-screen buttons, icons, menus, or other input or output elements are placed. Toolbars are seen in office suites, graphics editors, and web browsers...

, and a plug-in might embed a video player on the page. Since plug-ins and extensions both increase the utility of the original application, Mozilla

Mozilla

Mozilla is a term used in a number of ways in relation to the Mozilla.org project and the Mozilla Foundation, their defunct commercial predecessor Netscape Communications Corporation, and their related application software....

 uses the term "add-on" as an inclusive category of augmentation modules that consists of plug-ins, themes, and search engines.