An applet is a small computer program that performs a specific task or set of simple functions within an enclosed frame. It is typically embedded within another larger app or software platform and has limited functionality. Applets are not full-featured application programs and are intended to be easily accessible. They are often used in enterprise software, where they enable users to quickly access features without having to load a full app or switch windows. Applets can also be used for customizing the look and feel of desktop applications, providing the ability to personalize how the app looks and feels. In web development, applets allow developers to add interactive elements and animations to websites without needing too much additional coding.

The term "applet" is frequently used to refer to a Java applet, a program written in the Java programming language that is designed to be placed on a web page. Java applets were used to provide interactive features to web applications that historically could not be provided by HTML alone. They could capture mouse input and also had controls like buttons or check boxes. In response to the user action, an applet could change the provided graphic content. Applets were well suited for demonstration, visualization, and teaching, and there were online applet collections for studying various subjects, from physics to heart physiology.

Every applet is an extension of the java.applet.Applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context. These include methods that get applet parameters, get the network location of the HTML file that contains the applet, get the network location of the applet class directory, print a status message in the browser, fetch an image, fetch an audio clip, and play an audio clip. The <applet> tag is the basis for embedding an applet in an HTML file.