Information architecture (IA) is the art and science of organizing and labeling data including: websites, intranets, online communities, software, books and other mediums of information, to develop usability and structural aesthetics. Wikipedia
Information architecture is just one component in an emerging field known as user experience design. This recognises that a good digital service isn't just about functionality. It is about how people feel as they use a digital service, and about the way it does things, not just what it does.
At a micro level this can mean deciding that products on a search page should be ordered by price rather than by name. On a larger scale it could be reorganising the content on a site to support some clear tasks that users want to perform. On a strategic level, an information architect might get involved in determining the way that articles and metadata are placed into a content management system.
Louis Rosenfeld and Peter Morville defined the 'three circles of information architecture' as content, users and context of use.