Peter Morville and Louis Rosenfeld, who wrote the foundational text on the subject, defined IA through three overlapping circles: users (what they need and how they search), content (what exists and how it can be organised), and context (the business goals and technical constraints within which the system must operate). Good IA lives at the intersection of all three; IA designed without one of the circles is fragile.
Card sorting is the most reliable research method for IA decisions. It asks users to group content items they would expect to find together and label those groups — revealing the mental models users actually have, rather than the ones designers assume they have. The results are often surprising: taxonomies that seem logical to insiders are opaque to outsiders, and vice versa.
Navigation is IA made visible. The tabs, menus, sidebars, and breadcrumbs of an interface are the surfaces of the underlying information structure. This is why navigation design should follow IA design, not replace it. A beautiful navigation system built on a poorly conceived IA will send users confidently in the wrong direction.