What is ITIL?

First the short answer:

ITIL (Information Technology Infrastructure Library)

• Set of concepts and practices for IT Services, development and operation
• Most often used for setting up service support and delivery, i.e. incident, change, availability management

The long answer, with the following excerpted from Wikipedia, ITIL describes concepts and practices in the following areas:

1. Service Support

• Service Desk / Service Request Management
• Incident Management
• Problem Management
• Change Management
• Release Management

2. Service Delivery

• Service Level Management
• Capacity Management
• IT Service Continuity Management
• Availability Management
• Financial Management

3. ICT Infrastructure Management (Information and Communication Technology)

• ICT Design and Planning
• ICT Deployment
• ICT Operations
• ICT Technical Support

4. Security Management

• Control security process
• Plan, Service level agreements
• Implement, personnel, management, access control
• Evaluate, assessments, audits
• Maintain SLA, change requests

5. The Business Perspective

• Business Continuity Management
• Surviving Change
• Transformation of business practice through radical change
• Partnerships and outsourcing

6. Application Management

• Set of best practices proposed to improve the overall quality of IT software development and support through the life-cycle of software development projects, with particular attention to gathering and defining requirements that meet business objectives.

7. Software Asset Management

• Maintaining software license compliance
• Tracking inventory and software asset use
• Maintaining standard policies and procedures surrounding definition, deployment, configuration, use, and retirement of software assets and the Definitive Software Library.