What does ISO 27001 define?

The requirements for an information security management system (ISMS): for establishing, implementing, operating, monitoring, reviewing, maintaining and improving it with respect to the organisation's business risks, including the introduction of security controls tailored to the organisation.

ISO/IEC 27001 is the best-known and most widely used norm of the large ISO 27000 family. Its point is to ensure that an organisation selects sufficient and appropriate security controls that protect its information and give interested parties confidence. It applies to every kind of organisation: companies, government bodies, non-profits.

Note what it is not: it does not tell you how to build a secure network or system. It defines how security is managed, which is why it matters to the architect mainly as a source of requirements (next card).