IT Service Management (ITSM)
ITIL defines ITSM as a set of specialized organizational capabilities for providing value to customers in the form of services.
ITIL Glossary defines ITSM as the implementation and management of quality IT services that meet the needs of the business. IT service management is performed by IT service providers through an appropriate mix of people, process and information technology. See also service management.
Wikipedia provides a customer focused definition of ITSM as a discipline for managing information technology (IT) systems, philosophically centered on the customer's perspective of IT's contribution to the business. ITSM stands in deliberate contrast to technology-centered approaches to IT management and business interaction.
“Also termed service management thinking, service management is a systematic method for managing the offering, contracting and provisioning of services to customers, at a known quality, cost and designed experience. Service management ensures the desired results and customer satisfaction levels are achieved cost effectively, and is a means by which the customer experience and interaction with products, services, and the service provider organization is designed and managed. Service management is also a transformation method for any organization that wishes to operate as a service provider organization.” 
Difference between ITSM & ITIL
ITSM is an acronym for IT service management. It simply means how you manage the information systems that deliver value to your customers. Even if you’ve never heard the term ITSM, if you’re running IT systems, then you are doing ITSM. ITSM could include activities like planning and managing changes so they don’t cause disruption to the business, fixing things when they go wrong, or managing a budget to ensure you can pay the bills when they arrive. People who use the term ITSM tend to think of IT as a means of delivering valuable services to their customers, rather than as a way to manage technology—but even if you have a completely technical focus, your work still needs to be managed, and that’s what we call ITSM.
ITIL is the name of the world’s most widely recognized framework for ITSM. ITIL is a registered trademark of AXELOS, which owns a range of best practice solutions and their corresponding publications and exams. ITIL has been adopted by many organizations, and there are millions of certified ITIL practitioners worldwide.