What is a client?

A client is a computer or ap­pli­ca­tion that sends requests to a server. In doing so, it makes services and data provided by the server available to end users. Common client ap­pli­ca­tions include email programs and web browsers.

De­f­i­n­i­tion of clients

The term “client” means “customer”. In the IT sector, the terms “client ap­pli­ca­tions” and “client programs” are used. Here, client refers to an ap­pli­ca­tion (software) that has a user interface and com­mu­ni­cates with servers. When an end device, such as a personal computer, executes the client ap­pli­ca­tion, users can interact with it.

The client’s task is to prepare the correct server response for the re­quest­ing terminal. The client does this by sending requests to a server and then using and dis­play­ing the in­for­ma­tion and services provided by the server. For example, when a user wants to visit a par­tic­u­lar website, the web browser acts as a client, sending the request to a web server. The server responds and the browser outputs this response in the form of HTML5 and CSS in the browser window.

This in­ter­ac­tion between client and server can be described as the client-server model. The client-server model is regularly used to provide server services within a network.

Types of clients

There are different types of clients. In practice, mixed client types are also possible. Here are the most common ones:

Fat client

The fat client, also called thick client, is a fully equipped and powerful desktop PC. Local work on the client is possible with suf­fi­cient computing capacity. Char­ac­ter­is­tic features of the fat client are a pro­gram­ming interface, hardware, and a graphical interface. An example of a fat client is a PC on which Windows is installed as the operating system.

Thin client

A thin client is a computer or program that requires less local computing capacity, relying on the existence and ac­ces­si­bil­i­ty of a server. Due to the limited hardware resources, local work isn’t possible. The computing power is out­sourced to the server. The main task of a thin client is to take user input and send it directly to the server. An example is access to virtual desktops via a thin client.

Rich client

Rich clients, which are also known as smart clients, perform many tasks locally with the necessary computing resources. Out­sourc­ing a few tasks to a server is common. The ability to interact with a server also makes it easier to ad­min­is­ter a rich client.

Zero client

A zero client, also called an ultra-thin client, is a reduced kind of thin client. Its func­tion­al­i­ty is com­plete­ly dependent on its con­nec­tion to and com­mu­ni­ca­tion with a server. It has no local memory and is equipped with an operating system that has very few func­tion­al­i­ties. These are often used in vir­tu­al­ized desktop en­vi­ron­ments.

Examples of clients

Clients are used countless times every day through end devices. Some clients are used con­scious­ly by users, others run unnoticed in the back­ground.

Email client

The email client refers to the email program. The software lets users interact with email ac­tiv­i­ties. The email client enables writing, sending, receiving, reading, filtering and searching. Protocols used to fulfill these func­tion­al­i­ties are SMTP, IMAP and POP. Email clients are either installed as a stand­alone program or in­te­grat­ed into browser or smart­phone plugins. Well-known examples of email clients include Microsoft Outlook, Mozilla Thun­der­bird, and Apple Mail.

FTP client

The FTP client is used when you upload files to or download something from an FTP server. The client es­tab­lish­es an FTP-con­nec­tion to the server. The data exchange takes place over this con­nec­tion. You can find an overview of existing FTP clients in our article “9 FTP programs/FTP clients for Windows and Mac”.

SSH client

An SSH client is a program that es­tab­lish­es a secure and au­then­ti­cat­ed-SSH con­nec­tion to a server. The SSH protocol enables secure data transfers between two connected computers. SSH con­nec­tions are used for server ad­min­is­tra­tion, data transfer, and remote main­te­nance. A well-known SSH client is Windows’ PuTTY.

Web browser

Web browsers let you send requests to web servers and, for example, re­quest­ing and dis­play­ing webpages. The protocol that enables sending the requests is HTTP. Well-known and widely used web browsers include Google Chrome and Firefox.

DNS client

A DNS client is a client that runs unnoticed in the back­ground. Directly in­te­grat­ed into the TCP/IP network protocol, it converts a computer name or a web address into its cor­re­spond­ing IP address. This makes the client the in­ter­me­di­ary between the DNS server and the program that needs the server’s IP address.

The dif­fer­ence between a client and a server

The de­f­i­n­i­tion of a client, how clients work, and client use cases show that clients often rely on the server that they com­mu­ni­cate with. In many cases, the two operate together to perform a specific function. While servers and clients can both exist as physical computers, they differ sig­nif­i­cant­ly in their role and function in the client-server construct.

The client acts as a customer. It makes requests to a server and processes the server’s response as defined in the program. A client’s physical form is that of a computer or program.

The server provides a response to a client’s request. It provides the necessary services and resources to clients. It also handles com­mu­ni­ca­tion with the client by accepting requests and sending an ap­pro­pri­ate response based on its defined function.