According to the home of ITIL "ITIL® (the IT Infrastructure Library) is the most widely accepted approach to IT service management in the world. ITIL® provides a cohesive set of best practice, drawn from the public and private sectors internationally. It is supported by a comprehensive qualifications scheme, accredited training organisations, and implementation and assessment tools. The best practice processes promoted in ITIL® support and are supported by, the British Standards Institution's standard for IT service Management (BS15000)" [and now BS15000 is superceded by ISO20000].
I think this is a modest description. I would say "ITIL is the most widely accepted approach to IT management in the world". Theoretically service management is only one way to approach the job, but no-one has come up with a better one yet. Service management seeks to align IT with the business, a fancy way of saying they give the business what it needs not what IT wants to give it. This is just what everyone is trying to do these days, and ITIL is the best compilation of documentation on how to do that. It has been around for years (it came out of the UK government IT outsourcing inititiaives of the '80s and '90s) but its time has come.
The best general overview of ITIL available on the Web is this one