According to Wikipedia, ITIL is a "framework of best practice approaches intended to facilitate the delivery of high quality IT services. It outlines an extensive set of management procedures that are intended to support businesses in achieving value for money and quality in IT operations. These procedures are supplier independent and have been developed to provide guidance across the breadth of the IT infrastructure".
It was initially developed during the 1980's, by the CCTA, and was widely adopted in the 1990's. This in turn led to the development of a number of standards.
ISO/IEC 20000 is the international standard for ITSM, and aligned generally with ITIL. It comprises a 'specification' (part 1) and a 'code of practice' (part 2)
What is ITIL?
ITIL (the IT Infrastructure Library) is essentially a series of documents that are used to aid the implementation of a framework for IT Service Management. This customisable framework defines how Service Management is applied within an organisation.
Although ITIL was originally created by the CCTA, a UK Government agency, it is now being adopted and used across the world as the defacto standard for best practice in the provision of IT Service. Although the ITIL covers a number of areas, its main focus is on IT Service Management.