21. A method as recited in claim 19 , wherein an initial set of topics is pre-defined.
19. A method comprising: defining a set of topics for attachment to one or more information items of a plurality of information items, the defining being by at least one user, a user being a searcher who has provided registration information, the defining the topics including: accepting from at least one user at least one topic; and adding the topic to the set of topics, wherein the topics do not necessarily have a hierarchical structure; wherein each topic is for attachment to one or more information items of a plurality of information items, and wherein the topics that are attached to at least one information item are for searching by a particular searcher for one or more information items of the plurality of information items according to search request information provided by the particular searcher, search request information including at least one topic of the set of topics, the searching including identifying at least one information item that has attachments to the at least one topic of the search request information.