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Web Servers
Up to this point, most of the data you have worked with in your code has simply been generated and stored locally in variables.
While you'll always use variables to store and manipulate data while your program is running, most websites and apps use a web server to store, sort, and serve that data so that it sticks around for longer than a single session, and can be accessed by multiple devices.
Listening and Serving Data
Similar to how a server at a restaurant brings your food to the table, a web server serves web resources, such as web pages, images, and other data. The server is turned on and "listening" for inbound requests constantly so that the second it receives a new request, it can send an appropriate response.
The Server Is the Back-End
While the "front-end" of a website or web application is the device the user interacts with, the "back-end" is the server that keeps all the data housed in a central location.
A Server Is Just a Computer
"Server" is just the name we give to a computer that is taking on the role of serving data across a network connection. A good server is turned on and available 24 hours a day, 7 days a week. While your laptop can be used as a server, it makes more sense to use a computer in a data center that's designed to be up and running constantly.