Information Architecture (IA) is the practice of organizing and labeling content in such a way that it guides behavior and increases usability.
Designing the information architecture of a site or app involves techniques like story-mapping and creating visual vocabulary charts. IA helps us to understand the most important goals of the product or service, as well as the existing knowledge and assumptions of the end-user.
Good information architecture helps answer questions like: Does the interface of an application communicate its message? Can visitors find the information they want without help? Are visitors behaving as hoped and intended?
We talk about information architecture in the larger context of User Experience Design. It is a critical step in the process of planning and creating beautiful, user-friendly software.