Content Management System

it's an application or system that manages the dynamic content of a web page. think of posts of a blog which is a somewhat reputable template where only the content changes.
The CMS handles database connections, storing and fetching data from it. and it's all useful through a nice user interface, usually.

wordpress is the biggest CMS and started out for managing blog content but through extensibility grew to much more.

Headless CMS

Compared to a traditional CMS is a Content Management System that is one big monolith. where a single server handles rendering, the presentation, fetching data from DB, etc...

it is a way to describe an app that has it's backend separated from it front-end and they talk between each other with a REST API.
The CMS is only responsible for managing content and not the presentation

• Better performance than a regular CMS. faster loading.
• framework agnostic.
• scalable. front and back-end are decoupled.
• future proof bc it separates the content from the presentation.