An ad hoc search is the search type that SEOs are probably the most familiar with, even though we might not know the nomenclature for it. With an ad hoc search, the searcher's goal is to find as many relevant documents as possible regarding a topic. An ad hoc search is informational in nature, since the searcher is researching information about a subject. The searcher might or might not have previous knowledge about the topic but wants to read or learn more about it.
For example, suppose a searcher wishes to purchase a laptop but does not know what brand or type to purchase. The searcher might want to read reviews about different laptops before making a purchase. Some keyword phrases he might type into the search box can include:
types of laptops
digital laptop reviews
best samsung laptops
A known-item search is similar to an ad hoc search but the target of the search is a particular document, or a small set of documents, that the searcher knows to exist and wants to find again. In other words, with a known-item search, the searcher knows a particular web page exists but does not always know or remember where it is.
Or, interestingly enough, the searcher might know where the web page is but finds it easier to type all or part of the URL into the search box rather than the address bar.
Named page search
With a named page search, the web searcher wishes to go to a specific page within a website, a page that the searcher recalls seeing or visiting at a previous time. As an SEO professional, I often test the effectiveness of site optimization by doing named page searches.
For example, many SEOs use Google AdWords keyword research tool, but they do not always bookmark it or remember it when using the computer of a different person. So they might type the following keyword phrase into the search box:
Google Adwords keyword research tool
Clearly, this searcher wants to go directly to specific web page on Google's website. But I also like to do some advanced queries to see how well or poorly clients and competitors are optimizing their own sites. The queries I type use the following formats:
[keyword phrase] [company/organization name or abbreviation]
[keyword phrase] [part of URL with and without the .com or Top Level Domain]
As I continue to research, label, test and verify web searcher behaviors, I continually encounter these types of searches. As you can see, some of these searcher behaviors have overlapping definitions, which can be confusing.
Nevertheless, I maintain a list of different types of searcher goals so I can accommodate them in all of my client websites.