"The certitude that any book exists on the shelves of the library first led to elation, but soon the realization that it was unlikely to be found converted the feelings to a great depression. [Luis Borges: The Infinite Library]"

catalogues useful web sites and organizes them as a hierarchical list of web-addresses. By searching down the hierarchy the field is narrowed, although at each bottom leaf many entries remain, which can then be further narrowed by using keywords. Yahoo employs now a staff of about 200 people, each focusing on some area, who filter web pages that are submitted for review or located directly, and categorizes those pages into the existing classification. Some of the categories are dynamic, as recent events and entertainment, and aggregate information when a search is requested.
Note that Yahoo used to use InkToMe as its search engine provider, but switched in 2001 to Google.

automates the process, by surfing the web, creating indexes for terms extracted from the pages, and then using high-powered computers to report matches to the users. Except for limits due to access barriers, the volume of possibly relevant references is impressive. However, the result is typically quite poor in precision. Since the entire web is too large to be scanned frequently, references might be out of date, and when content has changed slightly, redundant references are presented. Context is ignored, so that when seeking, say, a song title incorporating the name of a town, information about the town is returned as well.

combines some of the features, and also keeps track of queries. If prior queries exist, those results are given priority. Searches are also broadened by using the ontology service of Wordnet [Miller:93]. The underlying notion is that customers can be classified, and that customers in the same class will share interests. However, asking similar queries and relating them to individual users is a limited notion, and leads only sometimes to significantly better results. Collecting personal information raises questions of privacy protection.

provides customer control over their profiles. Individuals submit information that will encourage businesses to provide them with information they want [Maes:94]. However, that information is aggregated to create clusters of similar consumers, protecting individual privacy. Business can use the system to forward information and advertisements that are appropriate to that cluster. There is a simplification of matching a person to a single customer role. Many persons have multiple roles. At times they may be a professional customer, seeking business information, and at other times they may pursue their sports hobby, and subsequently they may plan a vacation for their family. Unless these customer roles can be distinguished, the clustering of individuals is greatly weakened.

collects not only references, but also the webpages themselves. This allows Alexa to present information that has been deleted from the source files. Ancillary information about web pages is also provided, as the author organization, the extent and the number of links referring to this page. Such information helps the customer judge the quality of information on the page. Presenting web pages that have been deleted provides an archival service, although the content may be invalid. The creators of such webpages can request Alexa to stop showing them, for instance if the page contained serious errors or was libelous. Since the inverted links are made available one can also go to referencing sites.

ranks the importance of web pages according to the total importance of web pages that refer to it. This definition is circular, and Googleperforms the required iterative computation to estimate the scaled rank of all pages relative to each other. The effect is that often highly relevant information is returned first. It also looks for all matches to all terms, which reduces the volume greatly, but may miss relevant pages [PageB:98].

provides integration over diverse sources. By inspecting sources, their formats are discerned, and the information is placed into tables that then can be very effectively indexed. This technology is suitable for fields where there is sufficient demand, so that the customer needs can be understood and served, as advertisements for jobs, and searches for merchandise. later time by the same or a related application. For instance, a search for some movie, recorded in a cookie, can trigger an advertisement for a similar movie later. The use of cookies moves the storage of user-specific information to the user Accessing and parsing multiple sources allows, for instance, price comparisons to be produced. Vendors who wish to differentiate themselves based on the quality of their products (see Section 2.2.3) may dislike such comparisons.

is not an independent search engine, but a device used by many engines and applications to track users of use, the `freshness rejecting of cookies and applications that generate cookies.