Entity groups are a simple concept. Entities in App Engine have ancestors. For e.g. you could model Books and Authors.
Author: name->X, and Author: name->Y are two entities in the Author KIND. Book is another KIND (KIND is just a logical grouping of entities). The relationship between Books and Authors can be modeled as a ancestor-child relationship.
E.g.
Book: name->B1, B2 could have been written by Author: name->X. So you modeled them as:
Author: name->X is a parent of both Book: name->B1, B2. Similarly, Book: name->B3 is written by Author: name->Y and so could model that relationship as Author:name->Y is a parent of Book:name->B3.
When you try to transact on Books kind, you cannot transact on B1,B3 and B3 together. As they participate in different ancestor-child relationships. Every ancestor child relationship is an Entity group. You can only "lock" on one entity group at a time.
Hope this makes things a little clear.
