The distinction between information architecture and usability may seem like semantics, but there are significant differences between the two disciplines.
Though they are often discussed interchangeably, and practitioners are often well-versed in both, information architecture and usability differ in their scope and areas of focus.
The difference between information architecture and usability is important to understand because information architecture is more than just understanding what users want and need. A usability-only approach to information architecture is only one piece of the puzzle.
Information architecture problems often account for a large percentage of usability problems, but there are many other things unrelated to information architecture that have an impact on usability.