An applet is a small application designed to run within another application. While the term "applet" is sometimes used to describe small programs included with a computer's operating system, it usually refers to Java applets, or small applications written in the Java programming language.
Unlike ordinary applications, Java applets cannot be run directly by the operating system. Instead, they must run within the Java Runtime Environment (JRE), or within another program that includes a Java plug-in. If there is no JRE installed, Java applets will not run. Fortunately, Java is freely available for Windows, Mac, and Linux systems, which means you can easily download and install the appropriate JRE for your system. Since Java applets run within the JRE and are not executed by the operating system, they are crossplatform, meaning a single applet can run on Windows, Mac, and Linux systems.
While applets can serve as basic desktop applications, they have limited access to system resources and therefore are not ideal for complex programs. However, their small size and crossplatform nature make them suitable for Web-based applications. Examples of applets designed to run in web browsers include calculators, drawing programs, animations, and video games. Web-based applets can run in any browser on any operating system and long as the Java plug-in is installed.
During the early years of the Web, Java applets provided a way for webmasters to add interactive features that were not possible with basic HTML. However, in recent years, applets have been slowly replaced by newer technologies such as jQuery and HTML 5. Some browsers, like Google Chrome, no longer support the <applet> tag, and others, like Apple Safari, do not even enable Java by default. Since web developers cannot fully rely on Java support from web browsers, applets are no longer a common way to provide interactive content on the Web.