15. A method of producing a keyword hierarchy, the method being implemented at a computing device and comprising: mapping, at the computing device, closeness of a plurality of keywords extracted from a plurality of items, one to another, the closeness partially defining a relevancy of one of the plurality of keywords to another of the plurality of keywords and the closeness being generated by: for each one of the plurality of keywords: creating a hash table comprising information that includes: a number of the plurality of items having the one of the plurality of keywords as a root keyword; a number of a sum of ranks of the plurality of items that have the root keyword; a first structure pointing to the plurality of keywords that are likely included in the plurality of items that have the root keyword; and a second structure storing closeness value for each of the plurality of keywords in the first structure; assigning an inertia for each one of the plurality of keywords in the first structure; walking each one of the plurality of keywords in the first structure and adjusting its inertia based at least in part on likelihood of the plurality of items having the each one of the plurality of keywords as a keyword; calculating one or more steps that the each one of the plurality of keywords is away from the root keyword in distance and adjusting the inertia of the each one of the plurality of keywords based at least in part on the one or more steps; taking the each one of the plurality of keywords that are two or three steps away from the root keyword; and adding the current inertia of the each one of the plurality of keywords to the closeness value stored in the second structure each time the each one of the plurality of keywords in the first structure points back to the root keyword; forming, at the computing device, a plurality of groupings of the keywords based on the mapped closeness, wherein the plurality of groupings include two or more of the plurality of keywords into keyword phrases; combining the plurality of groupings based at least in part on closeness of the plurality of groupings until a predetermined number of groupings remain; and outputting said keyword phrases corresponding to the plurality of groupings for display at the computing device such that each of the keyword phrases, when output, is configured to be selectable to navigate to one or more corresponding said items that have the keyword phrase.
15. A method of producing a keyword hierarchy, the method being implemented at a computing device and comprising: mapping, at the computing device, closeness of a plurality of keywords extracted from a plurality of items, one to another, the closeness partially defining a relevancy of one of the plurality of keywords to another of the plurality of keywords and the closeness being generated by: for each one of the plurality of keywords: creating a hash table comprising information that includes: a number of the plurality of items having the one of the plurality of keywords as a root keyword; a number of a sum of ranks of the plurality of items that have the root keyword; a first structure pointing to the plurality of keywords that are likely included in the plurality of items that have the root keyword; and a second structure storing closeness value for each of the plurality of keywords in the first structure; assigning an inertia for each one of the plurality of keywords in the first structure; walking each one of the plurality of keywords in the first structure and adjusting its inertia based at least in part on likelihood of the plurality of items having the each one of the plurality of keywords as a keyword; calculating one or more steps that the each one of the plurality of keywords is away from the root keyword in distance and adjusting the inertia of the each one of the plurality of keywords based at least in part on the one or more steps; taking the each one of the plurality of keywords that are two or three steps away from the root keyword; and adding the current inertia of the each one of the plurality of keywords to the closeness value stored in the second structure each time the each one of the plurality of keywords in the first structure points back to the root keyword; forming, at the computing device, a plurality of groupings of the keywords based on the mapped closeness, wherein the plurality of groupings include two or more of the plurality of keywords into keyword phrases; combining the plurality of groupings based at least in part on closeness of the plurality of groupings until a predetermined number of groupings remain; and outputting said keyword phrases corresponding to the plurality of groupings for display at the computing device such that each of the keyword phrases, when output, is configured to be selectable to navigate to one or more corresponding said items that have the keyword phrase. 17. A method as described in claim 15 , wherein the combining is performed such that the number of keywords contained in each said grouping is approximately balanced, one to another.