Client and server

Frontend and backend are the roles. Client and server are the conversation.

A client is any program that asks for something. A server is any program that answers.

When you open instagram.com, your browser is the client. It says “give me the home page.” A program on Meta’s computers — the server — says “here it is.” That request and response is the whole pattern. Everything you do on the internet is some version of it.

The thing that trips people up

People hear “server” and picture a giant glowing machine in a data center. Sometimes it is. Often it isn’t.

Client and server aren’t kinds of computers. They’re roles in a conversation.

Your laptop is a client when it asks instagram.com for the feed. The exact same laptop becomes a server the moment another device on your wifi asks it to share a file. Same hardware, different role, different moment.

The word “server” gets used for the role and for the physical computer running the role’s code. Both are correct. Context tells you which.

How the conversation actually goes

Every interaction looks roughly like this:

The client sends a request. (“Show me my inbox.”)
The request travels over the internet to the server.
The server figures out the answer (often by reading a database — coming in two lessons).
The server sends back a response. (“Here are your 47 unread emails.”)
The client shows it to you.

This happens dozens of times when you scroll a single page. Every image, every comment, every new post that loads in is one of these round trips.

When you’ll hear this

Most of the time you’ll see it in compound words:

Web server — a server whose job is to answer requests from browsers.
Game server — a server that keeps the shared state of a multiplayer game.
Database server — a server whose job is to answer questions about stored data.
Client app — the program on the user’s device, doing the asking.

When Claude says “this server returns JSON” or “the client expects a token,” now you can mentally translate: the asker, the answerer, the thing being passed back and forth.

What’s next

These requests have to travel somewhere. They need addresses. Next lesson: the internet, but only the parts you actually need to know.