After loading class it appears in your class loader, and can be accessed like any other class.
There is a simple tutorial [here], that show how to load class from string.
In simplest case, you can load class, and hold it's Class object, using it to create objects dynamically. For field access or method invokation you can rely on Groovy dynamic nature.
There is no "cached source" or smth like that behind the scene and you can forget, from where your class is loaded. You can also cache classes, that are already compiled, and save them somewhere, as described [here]. It will drastically improve performance, if you need to load same class often.
But it will be better, to dig down in topic, because dynamic class loading is advanced Java/Groovy technique, it's whole infrastructure of chained classloaders, so it's better to refer documentation about them.
Links below may be helpful.
http://javarevisited.blogspot.ru/2012/12/how-classloader-works-in-java.html