The St. Louis Cardinals needed an upgrade at shortstop. Naturally it makes sense that they went after shortstop Rafael Furcal. Furcal is one of the top shortstops available despite a .197 batting average. That number is skewered to some extent because he spent the majority of the season on the DL and has only appeared in 37 games.
The Dodgers get AA outfielder Alex Castellanos for Furcal.

All in all-
Cardinals Acquire- SS Rafael Furcal
Dodgers Acquire- Alex Castellanos