As defined by Richard Saul Wurman (who coined the phrase) - "someone who analyses the patterns in data to help people find their own paths to knowledge" (...please correct that quote) (More info - Richard Saul Wurman Information Architects ISBN 1-8880-0138-0 (alternate, search))
Let's break that down:
Analyses patterns in data: I take this to mean the structures, topics, groupings, facets and metadata that may make up information. There are issues of domain knowledge here.
Help people find their own paths: IA is also about understanding how people access information, the keywords, labels and concepts they are familiar with. The interface that is designed to help them navigate. This implies knowledge of the audience and of the medium being employed.
They help design information spaces, particularly websites and similar hypermedia. They produce site maps, navigation systems, content inventories, metadata specs, thesauri, labelling systems, wire frames, [... please add more]
IAs have a range of techniques that can be used to get a better understanding of the best way to structure information for a given audience. These include:
The feature story from the April 2001 issue of Design Matters, the newsletter of the STC Information Design SIG, is now available online -- [Information Architecture: What's in a name?]
Learn what participants like Lou Rosenfeld, Jesse James Garrett, and Richard Saul Wurman have to say about whether there are differences between information architecture and information design.
Some blogs for more data:
and now a wiki:
Strangely enough, I consider WardCunningham an information architect, even though he doesn't seem to generate much of the the above output, mostly due to his restraint in inventing the wiki. Any geek programmer could add so much more functionality, but it would destroy the Wiki:WikiEssence in the process. See also Wiki:WikiDesignPrinciples. That careful restraint is something I'd like to think is part of the make up of an InformationArchitect?.