Information Architecture (IA) is something that any successful SEO should know about. Many people falsely believe that IA is all about navigation or that it actually is navigation. This is simply not true.
 
Navigation is about offering directions to the internet user to get from one place to another on a website. IA is a little more refined than this. IA is about offering directions in a particular way that is dependent upon who or what is asking for the directions.
 
IA, in essence, is all about the SEO understanding who the audience of a given website is, what they want and how these internet users in particular would go about searching for the information they desire.
 
In a quick snap-shot, IA covers the following main areas:

  1. Navigation
  2. Mental Model
  3. Taxonomy and Ontology

The above list is by no means exhaustive. There are many other areas of IA that a dedicated SEO should spend their time getting to know, but to begin with the above three highlighted areas provide a good place from which to start.
 
For more information about navigation, mental model and taxonomy