The ITIL Core consists of five publications. Each provides the guidance necessary for an integrated approach as required by the ISO/IEC 20000 standard specification:

1. Service Strategy
2. Service Design
3. Service Transition
4. Service Operation
5. Continual Service Improvement.

Each publication addresses capabilities having direct impact on a service provider’s performance. The structure of the core is in the form of a lifecycle. It is iterative and multidimensional. It ensures organizations are set up to leverage capabilities in one area for learning and improvements in others. The ITIL Core is expected to provide structure, stability and strength to service management capabilities with durable principles, methods and tools. This serves to protect investments and provide the necessary basis for measurement, learning and improvement.

The guidance in ITIL can be adapted for use in various business environments and organizational strategies. The ITIL Complementary Guidance provides flexibility to implement the Core in a diverse range of environments. Practitioners can select Complementary Guidance as needed to provide traction for the Core in a given business context, much like tyres are selected based on the type of automobile, purpose and road conditions. This is to increase the durability and portability of knowledge assets and to protect investments in service management capabilities.

Usually you can get a free ITIL Study Guide to prepare for ITIL Certification Exam.