ISO/IEC 20000 is an international standard for IT service management systems (Service Management System). It specifies what an organization must meet to provide its IT services effectively, measurably and with the possibility of continuous improvement - regardless of the size of the company or the field of business.
The standard is closely related to the ITIL methodology: while ISO 20000 specifies WHO a service management system should be meet, ITIL® offers HOW to achieve this in practice. That is why it makes sense to know and be able to combine both approaches. More detailed information on the connections between ITIL and ISO 20000 can be found in this article.