What is a server?
When you open an app, your phone doesn't have the whole internet inside it. It ASKS another computer for what it needs — and that other computer answers. That answering computer is a server. That's the entire idea. Everything else in system design is just "how do we keep answering when a LOT of people ask at once?"
A server is just a computer that waits to be asked
There's nothing magic about a server. It's a normal computer — often a plain one in a big room far away — running a program that does one thing forever: sit there, wait for a message, and reply. The computer that ASKS is called the client (your phone, your laptop, your browser). The computer that ANSWERS is the server. The message you send is the request. What comes back is the response. Ask, answer. Ask, answer. Millions of times a second.
Why split it up at all? Why not keep everything on my phone?
Two reasons. First, shared truth: if you and a friend are in the same game, the scores have to live in ONE place you both ask — not separately on each phone, or you'd disagree about who's winning. The server is that one shared place. Second, your phone is small. The server can be huge, always-on, and looked after by people whose job is to keep it running. Your phone can be cheap and just borrow that power by asking.
"Server" is a job, not a kind of machine
The same computer can be a client in one moment and a server in another — it depends on who's asking whom. "Server" describes the ROLE ("I answer requests") not a special golden box. When engineers say "add another server," they mean "run our answering-program on one more computer so we can answer more people at once" — which is exactly where the rest of system design begins.
Questions you might have
▸Is a server a special, expensive machine?
Not really. It's usually an ordinary computer (no screen, no keyboard) sitting in a data center. What makes it a "server" is the program it runs — one that waits for requests and answers them — not special hardware.
▸Where does the server actually live?
In a building full of computers called a data center, often in another city or country. When you tap your phone, your request travels over the internet to that building and the answer travels back — usually in a fraction of a second.
▸What's the difference between a client and a server?
The client is whoever ASKS (your phone). The server is whoever ANSWERS. The same computer can play either role at different times — it's about who's asking whom right now.
▸What happens if too many people ask at once?
That's THE big question of system design! One server can only answer so fast. When too many people ask, answers get slow or fail — so we add more servers, caches, and load balancers. Every other topic here is a tool for handling "a lot of people asking at once."