“Write once, run anywhere” (WORA), or sometimes Write once, run everywhere (WORE), is a slogan created by Sun Microsystems to illustrate the cross-platform benefits of the Java language. Ideally, this means Java can be developed on any device, compiled into a standard bytecode and be expected to run on any device equipped with a Java virtual machine (JVM). The installation of a JVM or Java interpreter on chips, devices or software packages has become an industry standard practice.
This means a programmer can develop code on a PC and can expect it to run on Java enabled cell phones, as well as on routers and mainframes equipped with Java, without any adjustments. This is intended to save software developers the effort of writing a different version of their software for each platform or operating system they intend to deploy on.
The catch is that since there are multiple JVM implementations, on top of a wide variety of different operating systems such as Windows, Linux, Solaris, NetWare, HP-UX, and Mac OS, there can be subtle differences in how a program may execute, which may require an application to be tested on various target platforms. This has given rise to the joke among Java developers, “Write Once, Debug Everywhere”. However, for a developer, the abstraction layer that Java provides is usually more convenient than recompiling software for each combination of operating system and architecture that it should run on and still represents a significant reduction in work when developing and supporting an application on multiple platforms. [From Wikipedia. All text is available under the terms of the GNU Free Documentation License.]