ITIL is organized into five core publications, that revolve around the service lifecycle. These provide best practice guidance for an integrated approach to IT service management.
The five core titles are:
Service Strategy
Service Design
Service Transition
Service Operation
Continual Service Improvement
To reflect this practice based approach, ITIL actually formally known as 'ITIL Service Management Practices'.
BACKGROUND - VERSION 2
The previous version of ITIL was organized into a series of sets, which themselves were divided into two main areas: service support and service delivery:
Service Support was the practice of those disciplines that enabled IT Services to be provided effectively.
Service Delivery covered the management of the IT services themselves. It involved a number of management practices to ensure that IT services were actually provided as agreed between the Service Provider and the Customer.
Each of these two areas contained a number of disciplines, which themselves stipulated the ITIL practices/requirements.