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.