What is WordPress plugin?

WordPress Plugin:

WordPress Plugin is a program written in PHP scripting, or a set of one or more functions, which works specific workspace or service to the WordPress WebBlog or on the website, which can be associated with the webplog using the Appear and the WordPress plugin application program interface (API).

The plugins contribution to the Popularity for the success and popularity of WordPress. With the plugins you can control your site's entire structure. If you want, you can control your site's / posts with plugins. 

WordPress plugins are used to add additional functions. Such as you want to create a form. But there is no facilities to create form for WordPress in the WordPress. Here you need the plugin.

A lot of famous is a plugin to create form. Contact Form 7. With this plugin you can create beautiful beautiful form. There are also many plugins for the same job.

To make thousands of plugins, https://codex.wordpress.org/plugins go to this link.