15. A system, comprising: a data processing apparatus; a tangible, non-transitory data store storing instructions executable by the data processing apparatus that upon execution by the data processing apparatus cause the data processing apparatus to perform operations comprising: identifying a plurality of content items that are each available to navigated to as a result selecting search results that correspond to respective ones of the textual content items; segmenting textual content from in each of a plurality of distinct ones of the content items into word strings, each word string including contiguous words in the textual content of a corresponding content item; classifying word strings having a frequency of occurrence that is less than a threshold frequency of occurrence as uncommon word strings, the frequency of occurrence for each word string being a number of occurrences of the word string in the plurality of distinct content items relative to a total number of word strings in the plurality of distinct content items; identifying matching uncommon word strings segmented from between pairs of distinct content items that each can be navigated to via search results returned to users in response to submission of search queries; and determining a rank score for each distinct content item based on the matching uncommon word strings, the rank score being a score indicative of the importance of each distinct content item relative to other distinct content items.
15. A system, comprising: a data processing apparatus; a tangible, non-transitory data store storing instructions executable by the data processing apparatus that upon execution by the data processing apparatus cause the data processing apparatus to perform operations comprising: identifying a plurality of content items that are each available to navigated to as a result selecting search results that correspond to respective ones of the textual content items; segmenting textual content from in each of a plurality of distinct ones of the content items into word strings, each word string including contiguous words in the textual content of a corresponding content item; classifying word strings having a frequency of occurrence that is less than a threshold frequency of occurrence as uncommon word strings, the frequency of occurrence for each word string being a number of occurrences of the word string in the plurality of distinct content items relative to a total number of word strings in the plurality of distinct content items; identifying matching uncommon word strings segmented from between pairs of distinct content items that each can be navigated to via search results returned to users in response to submission of search queries; and determining a rank score for each distinct content item based on the matching uncommon word strings, the rank score being a score indicative of the importance of each distinct content item relative to other distinct content items. 16. The system of claim 15 , wherein the operations further comprise: receiving a relevance score for each of the plurality of distinct content items, the relevance score being a measure of relevance of the distinct content item to a search query; ranking the plurality of distinct content items based on the rank score and the relevance score; and ordering search results for the distinct content items, the search results being ordered according to the ranking of the plurality of distinct content items.