GORM is a groovy layer on top of hibernate (not JPA), to put it simply. Grails uses Hibernate (by default) for it's ORM piece. GORM allows you to use the power of hibernate without a single XML file or annotation and it is really powerful. One of my favorite features is dynamic finders. For example, assume the following class:
The following methods (and the underlying HQL) are automatically available to you.
And any similar combination. There's so much more to GORM than this but you get the idea. GORM is generally used within Grails but I have heard of folks using it outside of Grails. Just hit google for information regarding this. By the way, there is a JPA Plugin for Grails that is basicallly GORM but using JPA (with hibernate as the implementation). I'm not sure I see the benefit but it is there if you want it.