Here at my new job, I need to be succinct and at least a little evangelical about what-is information architecture for web sites.
IMHO, IA is the science of creating a structure for 1) getting around a site intuitively, 2) prioritizing the paths that matter most, 3) telling a story about your work through your choices, and 4) making sure that people can move laterally and back “upward” in ways that are also intuitive.
It’s my experience that #2 and #4 usually get short shrift. And a purist might even argue that they’re not IA, but good site design. I believe I would disagree.
For a fiesta of definitions, see this cool compilation on Elegant Hack – which looks like a great resource in itself.