WordPress plugins are additional components that can be installed to extent the functionality of a WordPress site. There are over 40 thousand plugins currently available from WordPress plugin repository and the number is constantly growing.
Plugins can vary from small to large and can extend functionality in a minor way, like adding a featured image to the list of posts or in a drastic way like turning your site into a store.
Unfortunately there’s no standard as far as how WordPress plugins can be developed and therefore most WordPress issues are due to poorly written plugin code. It’s important to test plugins carefully on a development site before moving them into production environment.