The biggest change to the PMBOK 5 is the introduction of the  Knowledge Area for Stakeholder Management.  The PMBOK 5 know has 5 Process Groups, 10 Knowledge Areas and a total of 47 Processes.

The new Processes are:

• Plan Scope Management
• Plan Schedule Management
• Plan Cost Management
• Plan Stackholder Management
• Control Stackholders Management

A very nice summary of the changes is available here.