What are Wordpress Plugins and Widgets?

As I knew, both Widgets and plugins are used to customize our websites for better functions and performance in any cases. Both widgets and plugins, like themes are installed using the WordPress Dashboard. So, what is the difference between them and when should I use a plugin or widget?

Plugins are extensions or apps that can be installed.
Some example of plugins are image galleries, contact forms, brute force protection etc.

Widgets are like plugins, but they provide a way to arrange your sidebar content etc without having to alter your actual code of your website.
A widget is a plugin that interfaces with the people on the site. It shows something that the people can see and/or click on. a recent posts list for example.

To add:
Widgets usually are always live once on a page and can increase load time while plugins do not neccessory have to be live all the time and can be used at specific times when needed e.g. image compression plugins.

examples of a few plugins would be: EWWW Image Optimizer (by the way great image optimizer)

examples of a widget would be the recent post show on the side of your website.

Plugins are extension for WordPress when widget are just customizable blocks on your website.
If plugins can improve your website performances, most of them will make WordPress slower because a plugin add fonctionnality but require often some assets for that.
So my advice will be to limit plugin usage, there are must have plugins and some useless.
delete Any plugin you dont use and keep them update to have a safe WordPress

Plugins are ways to extend and add to the functionality that already exists in WordPress. The core of WordPress is designed to be lean and lightweight, to maximize flexibility and minimize code bloat. Plugins then offer custom functions and features so that each user can tailor their site to their specific needs.

WordPress Widgets add content and features to your Sidebars. Examples are the default widgets that come with WordPress; for post categories, tag clouds, navigation, search, etc. Plugins will often add their own widgets.

This is not as straight forward due to the concept being quite abstract. Think of a widget of extending the template dynamically by allowing a webmaster to deploy 3rd party plugins (as well as default plugins). Developers tend to use such functionality to display information that is not mission critical, non intrusive and easy to use / manipulate.