Background IT Service Management (ITSM) is a broad term describing any managed approach to providing IT services.
ITIL (Information Technology Infrastructure Library) is a specific body of ITSM guidance which originated with the United Kingdom's Office of Government Commerce (OGC) in the late 1980's. Since then, it has become the de facto world standard for Service Management good practice.
It's All About Organization
ITIL is about organization, not about technology. ITIL provides guidance to help IT teams work together more effectively and efficiently and to better align efforts with business needs.
ITIL is also about managing IT as a set of services aligned with business objectives. The core of ITIL is the Service Lifecycle, a framework for managing services from the strategy phase through retirement.