ISO 27001 is a specification for an ISMS. It is the security standard against which formal certification is available.
The ISO 27001 standard was first published in 2005. Its BSI forerunner, BS7799-2, was published previously in 2002, and formed the basis of the current standard.
The standard, again like BS7799-2, utilizes the well known PDCA (Plan-Do-Check-Act) approach, which is also used with ISO 9001 and ISO 22000, for example.
The Table of Contents of ISO27001
The first certification against ISO 27001 2005 were awarded in the first quarter of 2006.