Purpose:
Decouple an abstraction from its implementation so that the two can vary independently.

To think about this situation that an abstract class has two categories of inheritance. Each category can be a seperate hierarchical inheritance tree, if we derive all the subclasses from the abstract class, it will make complex when we need to get some sub-classes that contains facades from two categories. So we need seperate those subclasses into differenct categories, typically one into implementation hierarchy while another into abstraction hierarchy which can employ subclasses of implementation hierarchy. No matter how, the relationship between different hierarchies is a bridge. From this point, the bridge pattern design lets different hierarchies can evolve seperately.

The structure of the Adapter Pattern (object adapter) may look similar to the Bridge Pattern. However, the adapter is meant to change the interface of an existing object and is mainly intended to make unrelated classes work together.

reference:
book:"Design Patterns” Gamma et al.
http://en.wikipedia.org/wiki/Bridge_pattern
http://www.c-sharpcorner.com/Language/BridgePatternsinCSRVS.asp
http://www.codeproject.com/gen/design/bridge.asp