A WordPress theme is a collection of templates and styles that determine the appearance and layout of a WordPress website. Themes are responsible for the design and layout of the website, including the layout of pages, posts, and other content types.

WordPress extensions, also known as plugins, are add-ons that extend the functionality of a WordPress website. They can be used to add new features, integrate with other services, or customize the functionality of the website. Plugins can be used to add features such as contact forms, social media integration, and e-commerce functionality.

The main difference between WordPress themes and extensions is the purpose they serve. Themes are used to change the appearance of the website and control layout, while plugins are used to add new features and functionality to the website.

A WordPress theme is a must-have element of any website, as it sets the visual design and layout of the site, but it’s possible to have a functional website without any plugin installed. On the other hand, there are several features and functionality that are not available by default on WordPress, like SEO optimization, security, forms, etc. and for that, you’ll need to install the proper plugin.

It’s worth to mention that both themes and plugins can be found for free on the WordPress.org repository or can be purchased from third-party developers.