Learn in the textbookFundamentals of Service Management and Service Level Management
DefinitionITIL is a framework that systematically collects best practices (ベストプラクティス, ways of working that have proven successful) for IT service management.
ExplanationIt began as a set of books compiled by a UK government agency, and the word Library in its name refers to that collection. It presents model approaches to delivering and supporting IT services, and each organization adopts them to fit its own situation.
AnalogyLike a book of standard chess openings, it gathers moves that many earlier players tried with success, and you pick the ones that suit the position.