Plugins consist of add-in modules which you install on your WordPress server in order to implement additional functionality, instead of writing all the needed code yourself.
Where you might use a DLL with a Windows program - for example, to add a feature such as SSL support or an edit control into an existing application - you'd use a plugin with WordPress.
DLLs are usually written in a language such as C or C++ and compiled into native machine code; WordPress plugins are generally written in a mixture of JavaScript, PHP, HTML and CSS.
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