Introduction : ITIL stands for Information Technology Information Library. It is a series of five books that describe twenty-six processes and four functions in the latest version; Version 34. ITIL is a framework of best practices for IT service management.
ITIL is similar to COBIT but has some differences.
ITIL is a public framework that describes Best Practice in IT service management. It provides a framework for the governance of IT, the ‘service wrap’, and focuses on the continual measurement and improvement of the quality of IT service delivered, from both a business and a customer perspective.
The core of ITIL and IT Service Management is the book on Service Strategy. This is wrapped by the three books on Service Transition, Service Operation, and Service Design. The three books are wrapped by the fifth book, Continual Service Improvement.). See the figure below.

Further Readings and Tutorails
What is ITIL ?
COBIT ITIL and CMMI Tutorail
Wikipedia :: Information_Technology_Infrastructure_Library
UCISA :: ITIL Overview