ISO 20000 is the international standard for IT Service management.
Many people I talk to, usually describe both as a set of best practices for IT units to follow and this is true to a certain point. As ITIL does indeed work like that, the ISO guidelines are Industry Standards for which you have to pass certain tests and get certified as an organization pertaining to those qualities.
ISO 20000 itself is part of a much bigger picture, in that it aligns with ITIL, the IT Infrasture Library. This relationship is often illustrated via a diagram such as the one below: