The idea here is simple: Users will download Java bytecodes from the Internet and run them on their own machines. Java programs that work on web pages are called applets. To use an applet, you only need a Java-enabled web browser, which will execute the bytecodes for you. You need not install any software. You get the latest version of the program whenever you visit the web page containing the applet. Most importantly, thanks to the security of the virtual machine, you never need to worry about attacks from hostile code.
Inserting an applet into a web page works much like embedding an image. The applet becomes a part of the page, and the text flows around the space used for the applet. The point is, this image is alive. It reacts to user commands, changes its appearance, and exchanges data between the computer presenting the applet and the computer serving it.
Figure 1.1 shows the Jmol applet that displays molecular structures. By using the mouse, you can rotate and zoom each molecule to better understand its structure. At the time that applets were invented, this kind of direct manipulation was not achievable with web pages—there was only rudimentary JavaScript and no HTML canvas.
Figure 1.1 The Jmol applet
When applets first appeared, they created a huge amount of excitement. Many people believe that the lure of applets was responsible for the astonishing popularity of Java. However, the initial excitement soon turned into frustration. Various versions of the Netscape and Internet Explorer browsers ran different versions of Java, some of which were seriously outdated. This sorry situation made it increasingly difficult to develop applets that took advantage of the most current Java version. Instead, Adobe’s Flash technology became popular for achieving dynamic effects in the browser. Later, when Java was dogged by serious security issues, browsers and the Java browser plug-in became increasingly restrictive. Nowadays, it requires skill and dedication to get applets to work in your browser. For example, if you visit the Jmol web site at http://jnot.sourceforge.net/deno/aninoacids/, you will likely encounter a message exhorting you to configure your browser for allowing applets to run.
Source: Horstmann Cay S. (2019), Core Java. Volume I – Fundamentals, Pearson; 11th edition.