Plug-in is a term used for extensions that are developed by a third-party organisation for a specific program. For instance, if you are using a web browser to watch online videos, then you will need to install a plug-in called flash player. Flash player is a program that is not designed for a particular web browser. In fact, it is developed by Adobe, and is compatible with and used by all popular web browsers, such as Internet Explorer, Mozilla Firefox and Google Chrome, to play videos online.
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Add-on is also an extension that extends the functionality of a program. However, Add-ons are designed to work on specific programs. For instance, if you are using Mozilla Firefox, then you will have to install the Add-ons that are especially designed to be used on Firefox only. You cannot use Chrome Add-ons on Firefox, and vice versa. Toolbars are some the basic Add-ons used by different web browser. For instance, if you are an SEO specialist and you use different SEO tools regularly, then you can install a specific SEO toolbar, which includes Alexa Ranks, Google Page Rank calculator, back link calculator etc, on your browser. For that purpose, you will have to select a toolbar that is especially designed for the web browser you are using.
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