The CPU, or Central Processing Unit, is the active part of the computer. Its function is to execute programs that are coded in machine language and stored in the main memory (RAM) of the computer. It does this by repeating the fetch-and-execute cycle over and over; that is, it repeatedly fetches a machine language instruction from memory and executes it.

An asynchronous event is one that occurs at an unpredictable time outside the control of the program that the CPU is running. It is not "synchronized" with the program. An example would be when the user presses a key on the keyboard or clicks the mouse button. (These events generate "interrupts" that cause the CPU to interrupt what it is doing and to take some action to handle the asynchronous event. After handling the event, the CPU returns to what it was doing before it was interrupted.)

Compilers and interpreters have similar functions: They take a program written in some programming language and translate it into machine language. A compiler does the translation all at once. It produces a complete machine language program that can then be executed. An interpreter, on the other hand, just translates one instruction at a time, and then executes that instruction immediately. (Java uses a compiler to translate java programs into Java Bytecode, which is a machine language for the imaginary Java Virtual Machine. Java Bytecode programs are then executed by an interpreter.)

Programs written in the machine language of a given type of computer can be directly executed by the CPU of that type of computer. High-level language programs must be translated into machine language before they can be executed. (Machine language instructions are encoded as binary numbers that are meant to be used by a machine, not read or written by people. High-level languages use a syntax that is closer to human language.)

The Java compiler translates Java programs into a language called Java bytecode. Although bytecode is similar to machine language, it is not the machine language of any actual computer. A Java interpreter is used to run the compiled Java bytecode program. (Each type of computer needs its own Java bytecode interpreter, but all these interpreters interpret the same bytecode language.)

A subroutine is a set of instructions for performing some task that have been grouped together and given a name. Later, when that task needs to be performed, it is only necessary to call the subroutine by giving its name, rather than repeating the whole sequence of instructions.

An object consists of some data together with a set of subroutines that manipulate that data. (An object is a kind of "module," or self-contained entity that communicates with the rest of the world through a well-defined interface. An object should represent some coherent concept or real-world object.)

A variable is a memory location that has been given a name so that it can easily be referred to in a program. The variable holds a value, which must be of some specified type. The value can be changed during the course of the execution of the program.

A Java program can be compiled once into a Java Bytecode program. The compiled program can then be run on any computer that has an interpreter for the Java virtual machine. Other languages have to be re-compiled for each platform on which they are going to run. The point about Java is that it can be executed on many different types of computers without being recompiled.

The Internet is a network connecting millions of computers around the world. Computers connected to the Internet can communicate with each other. The Internet can be used for Email (which lets a user of one computer send a message to a user on another computer), file sharing (which is used to copy files between computers), and the World Wide Web (which lets a user view "pages" of information published on computers around the world).