Site architecture is the way a website is organized. It consists of defining the relationships between pages for proper URL structure, and identifying specialized third-party services and custom post types that can help expedite the initial development process, the proper layout of information, and designing the site so that users can easily access all options and features offered by the site.
A product architecture is a very similar plan to a site architecture, but with specialized plans to account for product types, tags, and collections that are unique for e-commerce organization and filtering.
A site architecture is rarely going to look the same as the user interface navigation, which will be streamlined to help visitors find the most helpful information.
A sitemap refers to the visual representation of the architecture, which is often used interchangeably at Infantree.