CLASSPATH tells Java where to search for programs

Where to look? The Java runtime system needs to know where to find programs that you want to run and libraries that are needed. It knows where the predefined Java packages are, but if you are using additional packages, you must tell specify where they are located.

CLASSPATH. Specifying where to search for additional libraries in Windows is easily done by seeting the environment variable CLASSPATH, which Java uses to see where it should find Java programs and libraries.

Automatic? Some IDEs tell the Java runtime system where to search, and double-clickable Jar (Java Archive) files don't have this problem

When do you need it? If you compile "by hand" (typing javac and java in a command window), or sometimes with TextPad or other editors.