Applets are small components that can reside within a web document.
They're just like any other component in your web document (such as paragraphs,
forms, tables, graphics, etc.).
They are different from other components because applets can have behavior, respond to user and/or system events and trigger events themselves. You can think of them as little applications embedded within your web document.
The advantages of applets over static pages/components are ample:
Technically applets need a Java Virtual Machine (JVM) to run. This virtual machine sits on top of a real machine ( e.g. an Intel, a PowerPC, an UltraSparc or yet another processor ). The JVM translates the Java bytecode (this is the machine code of the JVM ) in machine code for the appropriate processor.
The advantage is that only one component is required for all the different 'hardware' platforms.
Netscape has been incorporating Java from version 2, Microsoft from version 3. The monster share is for these two browsers, so almost all your visitors will have Java.