A browser extension is a computer program

Computer program

A computer program is a sequence of instructions written to perform a specified task with a computer. A computer requires programs to function, typically executing the program's instructions in a central processor. The program has an executable form that the computer can use directly to execute...

 that extends the functionality of a web browser in some way. Depending on the browser and the version, the term may be distinct from similar terms such as plug-in or add-on. 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 designed with the idea of being a small and simple web browser, delegating more advanced functions to Mozilla extensions

Extension (Mozilla)

]Add-ons are installable enhancements to the Mozilla Foundation's projects, and projects based on them. Add-ons allow the user to add or augment application features, use themes to their liking, and handle new types of content.-Extensions:...

. Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 Internet Explorer

Internet Explorer

Windows Internet Explorer is a series of graphical web browsers developed by Microsoft and included as part of the Microsoft Windows line of operating systems, starting in 1995. It was first released as part of the add-on package Plus! for Windows 95 that year...

 started supporting extensions from version 5

Internet Explorer 5

Microsoft Internet Explorer 5 was a graphical web browser released in March 1999 by Microsoft, primarily for Microsoft Windows, but initially with versions available for Apple Macintosh, Sun Solaris, and HP-UX. It was one of the main participants of the first browser war...

.. Google Chrome

Google Chrome

Google Chrome is a web browser developed by Google that uses the WebKit layout engine. It was first released as a beta version for Microsoft Windows on September 2, 2008, and the public stable release was on December 11, 2008. The name is derived from the graphical user interface frame, or...

 started supporting extensions from version 4. The 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,...

 desktop web browser supported extensions from version 10. The Apple Safari web browser started supporting native extensions from version 5.
The syntax for extensions may be quite different from browser to browser, or at least enough different that an extension working on a browser does not work on another one. As for search engine tools, an attempt to bypass this problem is the multitag strategy proposed by the project Mycroft, a database of search engine addons working on different browsers.