I ran across this definition of information architecture at IBM's developerWorks site and thought I'd share it here:

Design patterns for information architecture with DITA map domains

Information architecture

Information architecture can be summarized as the design discipline that organizes information and its navigation so an audience can acquire knowledge easily and efficiently. For instance, the information architecture of a Web site often provides a hierarchy of Web pages for drilling down from general to detailed information, different types of Web pages for different purposes such as news and documentation, and so on.

An information architecture is subliminal when it works well. The lack of information architecture is glaring when it works poorly. The user cannot find information or, even worse, cannot recognize or assimilate information when by chance it is encountered. You probably have experience with Web sites that are poorly organized or uneven in their approach, so that conventions learned in one part of the Web site have no application elsewhere. Extracting knowledge from such information resources is exhausting, and users quickly abandon the effort and seek the information elsewhere.

The same issues apply with equal force to other online information systems, such as help systems. The organization and navigation of the information has a dramatic impact on the user’s ability to acquire knowledge.