Java Virtual Machines operate on Java bytecode, which is normally (but not necessarily) generated from java source code; a JVM can also be used to implement programming languages other than Java. For example, Ada source code can be compiled to Java bytecode, which may then be executed by a JVM. JVMs can also be released by other companies besides Sun (the developer of Java) — JVMs using the "Java" trademark may be developed by other companies as long as they adhere to the JVM specification published by Sun.

Complete Java SCJP details at http://sunjavasnips.blogspot.com/