When asked about the differences and similarities between COBIT and ITIL, ISACA answered (as quoted directly here by Stephen Ibaraki) with the following:
COBIT and ITIL have been used by information technology professionals in the IT service management (ITSM) space for many years. Used together, COBIT and ITIL provide guidance for the governance and management of IT-related services by enterprises, whether those services are provided in-house or obtained from third parties such as service providers or business partners.
Enterprises need to govern and manage their information and related technology assets and resources, and those arrangements customarily include both internal and external services to satisfy specific stakeholder needs. COBIT 5 aims primarily to guide enterprises on the implementation, operation and, where required, improvement of their overall arrangements relating to governance and management of enterprise IT (GEIT). ITIL provides guidance and good practice for IT service providers for the execution of IT service management from the perspective of enabling business value.
COBIT: Big Picture, ITIL: Microscopic Directions
The quote goes on to explain how COBIT 5 “describes principles and enablers that support an enterprise in meeting stakeholder needs”, whereas ITIL is much more focused on the detail of those parts in service management. The post continues, explaining how COBIT is generally more broad in its scope of coverage and how ITIL is more focused on how to perform the work. Indeed, the post explains that COBIT is focused in on the “why” whereas ITIL is focused on the “how”.
Read the full blog post here: http://blogs.technet.com/b/cdnitmanagers/archive/2014/04/06/cobit-versus-itil.aspx