Client-side code runs on the user's web browser or device, while server-side code runs on a remote computer that hosts the website or application. This fundamental difference determines both what each can do and how they interact.
Client-side code (typically JavaScript, HTML, and CSS) executes locally on your machine after being downloaded from a server. It handles user interface interactions, form validation, animations, and displaying content. Because it runs in the browser, users can see and inspect it, and it has direct access to the user's device and display. However, client-side code cannot directly access secure databases or sensitive information, and its performance depends on the user's device and internet connection.
Server-side code (written in languages like Python, Java, Node.js, PHP, or C#) runs on the hosting server behind the scenes. It manages databases, authenticates users, processes sensitive business logic, and generates responses to send back to the client. Server-side code remains hidden from users and can securely handle private data and complex operations. However, it cannot directly manipulate what's displayed on the user's screen—it must send instructions to the client-side code to do that.
Modern web applications use both working together: the client-side code requests information and handles display, while the server-side code retrieves and processes that information securely. This separation enables responsive user experiences while protecting sensitive operations and data.