What is it?
Java is a programming language maintained by Oracle, which is available on all platforms. Source code is converted to an intermediate form called byte code, which is used by the compiler to execute the program on the machine. For every platform, this compiler runs in its own virtual machine, which is platform–dependent. However, for almost every platform, a Java virtual machine (called Java runtime environment or JRE) is available.
What is different?
Compared to compiled languages, Java code is easier to port to different platforms: in most cases code can be used on Windows, OSX or *nix machines without alterations. Only platform–dependent functionality that is dependent on available drivers may require alterations. Also interactions with the file system require special attention to prevent errors.