An entity may use other fine-grained classes to represent entity state.
Instances of these classes, unlike entity instances, do not have persistent
identity of their own. Instead, they exist only as part of the state of the
entity to which they belong. (From 2.0 Proposed Final Draft March 13, 2009
-- do I need to provide footnote or something?)
In short, an embeddable is a seperation of data into a Java class that
relies on the owning Entity for it's identity. Many(most) times an
embeddable resides in the same database row as the owning Entity.