In a nutshell this is the international standard fir ITSM (IT Service management). It's publication, in 2005, was a hugely significant development, as it is certainly going to become a major influence on the ITSM landscape.

ISO20000 comprises two documents:

ISO20000-1:2005
Information technology - Service management - Specification

ISO20000-2:2005
Information technology - Service management - Code of practice

It is the former of these two against which formal, organizational, certification is available. Again, the existance of a certification route for the enterprise is a major driver, as it potnetially acts as a market differentiator.

As with many international standards, the ISO documents are in fact based upon earlier publications by BSI. In this case, the original ITSM standards were BS15000-1 and BS15000-2.