A region is an area in a theme where content can be placed, defined by the theme itself. A block is a piece of content or functionality that gets placed into a region.
Think of regions as containers and blocks as the things that go inside them. A theme might define regions like header, sidebar, and footer. You then assign blocks like a login form, a menu, or a custom HTML block to those regions.
Regions are fixed by the theme, while blocks are flexible and can be moved, reordered, or given visibility rules based on user role, page, or language. The block system has been part of Drupal since the earliest versions and remains the primary way of assembling page layouts alongside newer tools like Layout Builder.