Maybe there are books on this subject, but I'm having a hard time of late keeping up on my reading. All the same, during a discussion I was having with a coworker the other day, I had a clarifying moment. I already knew that searching, browsing, and navigating were similar, but not the same. But I hadn't really given thought to how they were different and how those differences applied to site/application development. This is how I see it:

• The user knows what they're looking for, where it is, and how to get there
• No decisions or back tracking en route
• Highest accuracy of approach (no guessing)

• The user knows what they're looking for, doesn't know precisely where it is, but knows how to find it
• Some decision making and the occasional back track
• Accuracy dependent on complexity of structure

• The user might know what they're looking for, doesn't know where it is, and doesn't know how to get there
• Every unsuccessful search is a back track (the user is always going from zero). Much decision making: how to phrase the search, and if the search results don't yield what the user is looking for, why not?
• Greatly variable accuracy: dependent on search complexity, search engine quality, search method (tag search, content search, category search...), etc.

In a broader sense, as the sum knowledge about the target and the containing system decreases, the method for reaching a destination slides from Navigating to Browsing to Searching. A site with no apparent structure will immediately jump the user to Searching. A site with a complex structure will quickly jump the user from Browsing to Searching. It's a baby bear scenario (if I may coin a phrase). For ideal usability, you want your site/app to provide simple browsing that is unlikely to break down into searching and searching that is reliable as a means to a destination, but primarily you want your user to know what they're looking for, where they're going, and how to get there. This is different between site design and application design because in site design, the user (generally) doesn't have control of the content or structure. In an application, however, the user usually has control of the content, and—depending on the app—at least some control of the structure. For example, applications that work with files saved to a user's file system benefit by passing the data structure management to existing tools available to the user to move/copy/rename/organize those files (e.g. Windows Explorer). The user knows where things are because they have a hand in establishing a structure that makes sense to them. So when designing a site or an application, consider the structure of your content and how much you want to help your users. How can you structure your site/app to keep your users in the Navigating< ->Browsing range? Can you do this successfully enough that you don't need a search? Maybe you're inclined to keep your site/app as thin as possible and provide very little or no Browsing, relying on the search as the primary navigation tool.