Backend Development

Backend Development is also known as server-side development. It is everything that the users don't see and contains behind-the-scenes activities that occur when performing any action on a website. It focuses primarily on databases, backend logic, APIs, and Servers. Thus, it is known as the backend.
Another way to understand the difference between the two is to understand the knowledge required of a front-end vs. a back-end software developer. The list below focuses on web development as an example.