Here is what they say about it on their website:
Groovy is designed to help you get things done on the Java platform in a quicker, more concise and fun way – bringing the power of Python and Ruby inside the Java platform.
Groovy can be used as an alternative compiler to javac to generate standard Java bytecode to be used by any Java project or it can be used dynamically as an alternative language such as for scripting Java objects, templating or writing unit test cases.
Overall it sounds rather interesting. It seems that it feel the niche between Java and Perl/Python/Ruby, providing an acceptable compromise for all those people participating in the holy wars. :)