Action chaining is a technique to handle the infamous
"MIXED_DML_OPERATION, DML operation on setup object is not permitted
Error" problem.

You may not know it but there are two types of objects that
you can interact with on the platform. One is "setup" object and the
other is a "non-setup" object. A "setup" object is one that must be
edited from the setup or builder area of the platform. These object
include the User object, Ogranization object, Email templates and so
on.

See this article for details DML operations of a non-setup sObject and a setup sObject

Read the full article and check out the source code at Visualforce Action Chaining

See you at Dreamforce!!

Dave Carroll