Backend

The backend is the part of an application that runs on a server rather than in the visitor's browser, handling the work the browser cannot safely do on its own, such as storing data, checking who is signed in, and holding secret keys.

The backend is the part of an application that runs on a server rather than in the visitor's browser, doing the work the browser cannot safely do on its own. It receives requests from the frontend, decides what should happen, reads or writes the stored data, and sends an answer back. Take a to-do app: when a visitor types a task and clicks save, the browser sends that text to the backend, which confirms the visitor is signed in, writes the task to the database under their account, and returns the updated list. The browser only shows the result. Anything that must stay private, like an account password or an outside service key, lives on the backend so it never reaches the visitor's machine.

Builder example

When you ask an AI to add a feature that touches stored data or a paid service, you are asking it to write backend code. A password reset is a good example: the browser collects the email, but the backend has to verify the account exists, generate a one-time link, and send the message through an email service using a secret key. If that key sat in the frontend, anyone could read it from their browser and send mail as you. Knowing which side a task belongs on tells you where a bug or a leak can hide.

Common confusion: The frontend is what the visitor sees and clicks; the backend is the server-side code they never see directly. People often assume a slow or broken page is a frontend problem, when the real cause is backend code timing out or a database query failing. Open the server logs before you change the layout.