What is plugins....what type of plugins used in web development?

Answer 1

The first paragraph in this link should give you a good idea:

https://en.wikipedia.org/wiki/Plug-in_(computing)

Plug-ins in the context of Web development usually refers to browser extensions.

https://en.wikipedia.org/wiki/Browser_extension

Answer 2

✓A plugin is a software add-on that is installed on a program, enhancing its capabilities. For example, if you wanted to watch a video on a website, you may need a plugin to play it because your browser doesn't have the tools it needs. 

✓Mostly plugin are used on WordPress to give extra features to to WordPress