A Service is defined by the IT Infrastructure Library (ITIL) as a service that delivers value by enabling outcomes that users or business functions want to achieve without owning specific costs or risks.
Tier A: Internal IT Services for example, technical services such as hosting, security, and storage.
Tier B: Business Aligned Services that are arranged in two groups, Commodity Services needed to run the business and Competitive Advantage Services to help the business be competitive.
Tier C: End User Aligned are services used by employees which are measured by user experience and cost.
The services themselves are organized by Service Type, Service Category, Service, and Service Offering.