Java is a programming language and free computing platform. It is the underlying technology that powers state-of-the-art solutions, like Liferay Portal. While portal works with Java version 6, it is recommended to use the latest version, Java 7.
If you do not have Java installed in your environment; don't worry! Portal bundle archive comes with Java and you will be able to run and use the portal.
Alternatively, you may download and install Java in your environment by yourself. Visit http://java.com and choose the Java installation according to your operating system. Just follow the online instructions and verify if the Java has been successfully installed.