Using the analogy of your store

Imagine that you own a bakery. The actual products are the content. Your products (or content) are created using your post types (types of content in WordPress). 

The stands, cash register, display showcases, and shelves are functionalities. It allows you to store and display your content in a certain way. They have limits (such as weight and size) of the products being displayed. WordPress’s built-in system and add-ons manage this.

Finally, how your theme controls your shelves, your walls, and your store lighting look. Some themes can also come with shelves, a cash register system as part of the package, or other functionalities.