A back-end server is a part of the back-end process, which usually consists of three parts: a server, an application and a database. The back end is where the technical processes happen, as opposed to the front end, which is usually where the user's interaction occurs.
When a website is accessed, the user is interacting with the front end. The website's application then stores the information entered into the website in a database that was created on the server. The information remains on the server; if the user returns to the website, the user's information is retrievable.
The server houses the information, like a hard drive does for an individual computer. While the hard drive only services one computer, the server services an entire website and therefore all the information being fed to it across the Internet.
Programming languages such as Python, Ruby and PHP are usually used for the back-end processes. A user does not come into contact with the back-end process. A general rule is that the front end (or the client side of the application) is any component that is manipulated by the user. The back end, or server side, is code that remains on the server.