On the web, information architecture involves the design of navigation, organization, labeling, and search systems. The art and science of organizing information to help people effectively fulfill their information needs.
Information architecture involves investigation, analysis, design and implementation.
There are other definitions, but this is the one put forward by Argus and reflects how many people practice information architecture. It is important to note that the Argus definition of IA has it's roots in library and information science. But Argus insisted that library and information science had much to offer the world, especially the web. IA has moved beyond these library roots to include elements of information design, interaction design, usability engineering, ethnography, and other fields. And Argus had added many of these elements to their consulting team.