As implied by the name, the front-end is the “face” of the website, or what the end-user sees when visiting the site. The front-end is commonly referred to as website design.
The website’s back-end, or the server-side, is where the work happens to update what the user sees in the front-end in real-time. The back-end is commonly referred to as website development.
The back-end of a website houses the server that hosts the website, storing all of the data that runs the front-end in a database. Without this stored data, there would be no front-end.