You can use Groovy classes from Java and Java classes from Groovy. This forum has 'scripting' in its topic, but that is one part of Groovy. You can use Groovy to create classes instead of Java, when appropriate. And of course, you need the groovy jar in your classpath.
Using a Java class from Groovy is exactly like using a Java class from Java. There is no difference. It's fully transparent.
Using a Groovy class from Java requires you to either - groovyc your Groovy class and put the resulting class files on the classpath - or use one of the several integration mechanisms - Eval, GroovyShell, GroovyScriptEngine, GroovyClassLoader - BSF or JSR-223 - Spring
The several mechanisms are there to provide support the simplest cases of evaluating a simple string of Groovy code up to the point where you have Groovy classes delivered transparently through the class loader.