In this interview with Peter Morville he gives a nice definition of Information Architecture, part of which reads "An emerging discipline and community of practice focused on bringing principles of design and architecture to the digital landscape".

When I walked into the Waterstones bookshop on Piccadilly with Phil a few weeks ago, directly inside the front door I found myself in a small lobby. Made of glass, the lobby sat as a bubble inside the grand entrance to the shop and could be exited (into the bookshop itself) by two doors on the right and left. Standing facing forward: the glass encouraged me to look directly into the bookshop, the view obscured by posters. Now my attention was directly ahead, the lobby doors were just outside my peripheral vision. After looking around and leaving the lobby, I was pointed at the corner of the room, no longer at the grand shop that was ahead of me. Naturally, I thought this was ridiculous. A website would never be constructed like this, I said. My gaze was being drawn away from the navigation options, the posters ahead of me (in prime real estate) weren't targetted or the most important thing I should see, and obscured the main proposition. Going deeper I was left directed away from the shop floorplan (site map) and a confused user. Why don't they learn how to use geography and location like Information Architects learn how to use the www?, I said, Information Architects know how to move people around a site, how to design it for comfort and familiarity. Designers in the real world could learn a lot from websites.

It was a full ten minutes before it occurred to me that the designers of the shop were in fact architects, and that the term Information Architecture was a derivation from that.