There’s often confusion over the distinction between information architecture, navigation design, and graphic design.  One of my favorite books that explains what these terms are and the distinctions is the book, The Design of Sites, by Douglas K. Van Duyne, James A. Landay, Jason I. Hong.

Van Duyne, Landay, and Hong define the terms as follows:

• Information Architecture - Identifying, structuring, and presenting groups of related content in a logical manner.
• Navigation Design - Designing methods so that customers can find their way around the information structure.
• Graphic Design - Developing the visual communication of information, using elements such as color, images, typography, and layout.

Note that information and navigation design are typically done before graphic design. 

Also note that the authors mention that there's often a debate in the design community about the boundaries between information architecture and information design.  They point out that information architecture focuses more on structure and language, while information design focuses on presentation and perception.  At the end of the day, the key point is that the two disciplines are about helping customers find, understand, and manage complex information.

It’s also worth knowing the terms “Information Model” and “Data Models” since they often come up in discussions regarding information architecture:

• Information Model -    A model of the concepts, relationships, constraints, rules, and operations for a given domain, and it can provide a sharable, stable, and organized structure of information requirements.  See Information Model (Wikipedia)
• Data Models – Describes how the data are represented and accessed and defines the data elements and relationships among data elements for a given domain.  See Data Model (Wikpedia)