ISO 27001 is the direct replacement for BS7799-2:2002. Basically, it defines an information security management system, known as an ISMS. In the words of the standard itself, it is intended to "provide a model for establishing, implementing, operating, monitoring, reviewing, maintaining and imrpoving an information security management system.
It offers a process driven approach, known as plan-do-act=check PDCA), which is described elsewhere on this site. This in turn reflects the principles set out in the OECD Guidelines, published in 2002.
Note that it is this information security standard against which third party audit and certification is focused, rather than ISO 17799, which offers a more supportive role.
Although still in the FDIS phase, the contents of ISO 27001 are as follows:
For those organizations already certified against BS7799, there will be a recognized transition period for conversion to the new standard (ISO 27001).