ISO 14001:2004 sets out the criteria for an environmental management system and can be certified to. It does not state requirements for environmental performance, but maps out a framework that a company or organization can follow to set up an effective environmental management system.
ISO 14001 is often seen as the corner stone standard of the ISO 14000 series. However, it is not only the most well known, but is the only ISO 14000 standard against which it is currently possible to be certified by an external certification authority. Having stated this, it does not itself state specific environmental performance criteria.