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What is a client (computing)

Also known as: client-side, user agent, web client, client server

Definition

A client, in computing, is the program, such as a browser or an app, that sends a request to a server and shows you the answer. It is not your paying customer.

Why it matters

Anything that runs on the client can be changed by the person holding the device, so prices, discounts and permissions must be checked again on the server.

Common mistakes

Trusting the client for security, so a user who edits the page in the browser can change a price or skip a permission check.

Related terms

Worked exampleIllustration

A booking request: who is the client, who is the server

The address she opens

  • The connection is encrypted, which is not proof the site is safehttps://
  • The domain: which server to askwww.example.com
  • The path: which page on that server/booking

Client

Her phone's browser

Server

The clinic's server

Steps

  1. She opens the clinic's booking page, and her phone's browser, the client, sends a request.

  2. The clinic's server returns the page, and the browser draws it on her screen.

  3. She types a phone number with a letter in it, and the form flags it at once, for convenience only.

  4. On submit, the server checks the number and the free slot again, because the check in step 3 can be bypassed.

  5. The clinic's server now acts as a client and asks the messaging provider's server to send her the confirmation.

Client and server are roles, and anything the client decides is checked again by the server.

Illustration

How it works

Client, Request, Server, ResponseClientbrowser orappServerstores andprocessesRequestResponse

The client asks and shows. It shows only what the server sends back, and anything it checks, the server checks again.

The client is the device that asks: it sends the request and displays the answer the server returns.

Definition

A client is a program that sends requests to a server and presents the response, and loosely the phone or computer it runs on. The HTTP standard, RFC 9110, defines an HTTP client as "a program that establishes a connection to a server for the purpose of sending one or more HTTP requests", and a server as a program that accepts those connections and sends responses. Client and server are roles, not machine types: the same program can be a client in one exchange and a server in another, so your site's back-end becomes a client when it calls a payment provider's API. The standard's wider term is user agent, "any of the various client programs that initiate a request", which covers browsers, apps, command line tools and robots, not only a person at a browser. Clients you already use: a browser, the WhatsApp app, the email app on your phone, and a card terminal app talking to the payment company's server.

Why it matters

A form can block a letter typed into the phone field for convenience, but anyone can bypass that in the browser, which is why the server checks the number again before saving it. It is a security rule you can act on yourself when you brief a developer. The client also decides how your site feels: it has to work on small screens, older phones and slow mobile connections.

Directly related: Server, Front-end, Full-stack.

Improvement tips

  • Check prices, discounts and permissions again on the server, whatever the browser already checked.
  • Test the site on a small, older phone and on a slow mobile connection, not only on the office computer.
  • Keep client side checks for convenience, such as flagging a mistyped phone number at once, and never rely on them for safety.
  • Send the client only the data the screen needs, so pages load faster and nothing private travels by mistake.

Common mistakes

  • Sending too much JavaScript to the client, so older phones lag.
  • Assuming every visitor has a fast, stable connection.
  • Reading "client" in a technical document as the paying customer, and missing that it means a program.
Client vs Server

Related terms

TechnologyBeginner

Server

A server is a computer, or a program on one, that receives requests from other programs, such as a visitor's browser, processes them and sends back an answer.

TechnologyBeginner

Front-end

Front-end is the part of a website or app that users see and use, such as text, buttons, forms and images, built with HTML, CSS and JavaScript.

TechnologyIntermediate

Full-stack

Full-stack describes a developer, or a team, who builds both what users see (the front-end) and the server and database behind it (the back-end).

TechnologyIntermediate

API

An API (application programming interface) is the agreed way one program asks another for data or an action and gets an answer, so your tools can work together.

TechnologyIntermediate

Authentication

Authentication is the check that a user is who they claim to be, using something they know, something they have or something they are.

Quick check

In client-server architecture, what is a client?

Choose an answer

Frequently asked questions

Do I need to understand the client before I start my business?
Only the basic idea: the client is the browser or app your customers use to reach your service. It reminds you to test on different phones and browsers before launch, and it helps you read a developer's quote, where "client side" means the part that runs on the customer's device, not the customer.
When does the client first become relevant for a new business?
The day you start building a website or app. Your pages have to load and display correctly in Chrome, Safari, Edge and the browsers built into WhatsApp and Instagram, on large and small screens.
How does client side performance affect my marketing?
A page that makes the phone work hard loads slowly on older devices, and visitors from your ads leave before it appears. Google uses page experience, including Core Web Vitals, as one ranking signal among many, while relevance matters more. The front-end page explains the thresholds.
Should I build a client side app or a traditional web page?
A traditional page, which the server sends ready, is faster to build and easier for search engines to read reliably, so it suits a marketing site. A client side app feels smoother for interactive tools but costs more to build. Choose by what the visitor does: reads and contacts you, or works inside the tool.
Why does the client matter for a business already running?
Your customers meet your business through a client, a browser or an app. If the app is slow on their phones or breaks after a browser update, they leave for a competitor without telling you. Test after every major browser or phone system update.
What goes wrong when a business trusts the client?
Anything the browser checks, a user can change. If the discount is calculated only on the page, or the permission check runs only in the app, someone can edit the request and pay less or see what they should not. The server has to check again.
How do I improve loading times without stopping day to day work?
Start with the images, compression and caching. A developer or your hosting provider can do this without changing the design, and the site feels faster for every visitor at once.
How do I know if customers are hitting errors in their browsers?
Error tracking software alerts your developer when pages fail in a customer's browser. Your analytics can also show visitors on a particular phone model or browser leaving much faster than the rest.
What does client actually mean in plain words?
It is the program that asks, such as Safari on an iPhone, Chrome on a laptop or the WhatsApp app. It sends a request to a server and shows you what comes back. It is not the client who pays you, only the same word.
Is client side code safe for my personal data?
Code in the browser runs in a protected space, but what matters for your data is the connection and the site. Check that the address starts with https and that the domain is the one you expect. The icon differs by browser and is not proof the site is safe, because HTTPS encrypts the connection and says nothing about who runs the site.
Do I need a developer to optimise client performance on my site?
For a standard site, good hosting and a caching or image plugin cover the basics. For a custom web app, a front-end developer should audit the code and cut what the phone has to download.
What is the difference between client side and server side?
Client side runs on the visitor's phone or computer, in the browser or the app. Server side runs on the business's server, where the database is read, payments are processed and every important check is made again.

Sources: IETF, RFC 9110, HTTP Semantics, 2022, sections 3.3 and 3.5, Chromium Blog, An Update on the Lock Icon, 2 May 2023, Google Search Central, Understanding page experience in Google Search results (developers.google.com), read 23 September 2026

Last reviewed: 2026-09-23

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Anna, Beautician

Anna

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After I finished a cosmetology course at a leading company, I was confused. I didn't know how to open a business, what to sell from home, or how to bring in clients. Since I reached out to Mobius, everything has changed. We built an organized plan with clear steps and no unnecessary risks. Today I see results, and I'm growing and developing every month.

Mark, Massage therapist, Gan Yavne

Mark

Massage therapist, Gan Yavne

When I finished a massage course, I imagined a different world. I found out it's a hard field, and financial stability isn't easy to find. When I came to Mobius, something changed. We started an organized process and built a business plan. Without Mobius? I probably would have given up. Today I believe in myself and in my own path.

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