To summarize a CROSS APPLY, it's a correlated subquery much like you might find in a subquery within a SELECT list except that it's capable of returning more than one row. CROSS APPLY is like an inner join against the subquery and OUTER APPLY is like a LEFT OUTER JOIN away from the subquery.
Paul white wrote a couple of great articles on the subject right here on SSC.
http://www.sqlservercentral.com/articles/APPLY/69953/
http://www.sqlservercentral.com/articles/APPLY/69954/
--Jeff Moden
Change is inevitable... Change for the better is not.