Every website has an information architecture, though few are planned intentionally. Information Architecture is the organization of information. The term is relatively modern (1976), but encompasses concepts from a variety of existing fields including classification systems such as the Dewey Decimal System invented a hundred years prior (1876).
Information Architecture concerns itself with the myriad mechanisms enabling users to navigate information and — to some extent — the structure of that information. Just as a library has a Dewey Decimal System, a book described by that system may have its own Table of Contents, Index, and Glossary.
In the context of the web, IA typically encompasses:
Deeper research in this field reveals roots adjacent to category theory and cognitive linguistics. In order to organize information in a way that is obvious to humans, one must understand the manner in which we as humans organize information within our own minds; a fascinating field of study that is both culturally subjective and constantly evolving.