With the file extension class, CLASS files of the Java programming language are identified. (Classes Are Programming Constructs) Source code files (file extension java) are compiled into so-called (virtual) machine-readable CLASS files and contain bytecode. This bytecode is only executable in a Java virtual machine (JVM). Since there are JVMs for many different systems, Java is considered a platform-independent programming language.
Each CLASS present in a source code is compiled into a separate CLASS file, which is usually summarized in a jar file. If a JVM is installed, java files can be compiled with the javac command. Alternatively one can also use a Java IDE (integrated development environment), for example Eclipse for Java.
Since there are different versions of the JVMs (1.4, 1.5 and 1.6), it may happen that a CLASS file on certain JVMs may not work. Note that JVMs CLASS can only execute files but not generate them.
The developer of Java - Sun Microsystems - has since been bought by Oracle. A common name for the JVMs is the Java Runtime Environment (JRE).