Interesting question eh ? Nobody would have any issues in defining a CEO or CFO role. But C.I.O ? Allan Holmes calls for two types of CIO's, strategic and tactical. "Strategic CIO who would focus on creating value through technology, and a tactical CIO whose primary goal would be to run an efficient IT shop." Peter Weill of Sloan MIT says "The CIO’s expertise is in communicating to business colleagues the importance of IT—and more important, getting the governance right." Peter seems to point us to the CIO as a business partner, some one who can tell the business as to what is possible with technology.
              Chaim Yudkowsky provides the following guidelines for a CIO role :
Manage and safeguard key data, communications, hardware and software.
Provide input and feedback in the overall business strategy and budgeting process, not only for IT. 
Help improve process efficiency and reduce cost using technology. 
Be an active part of an executive team with a mission of profitability and productivity.

Most CIOs come from a predominantly IT background and according to CIO Insight : 65% IT, 9% business, 26% hybrid. Jeff Wacker of EDS has an interesting view on what CIO's should not be : Chief Inertia Officer , Chief Impediment Officer, Chief Inefficiency Officer.