It is not always easy to make up a clear classification of a search engine or directory. Below, you will find an overview of the classifications used on this platform and their specifications.
Updated info. SeEn
Crawler based algorithmic SeEn
Manually by an editorial team selected sites SeEn
Meta, multi or parallel SeEn
Deep web SeEn
SeEn based on social behavior (also called social media SeEn, which should not be confused with the following)
Manually selected sites by a team of volunteers SeEn (also called social media or Q&A SeEn)
Mobil web SeEn
Real time SeEn
Blended result SeEn
Visual result SeEn
SeEn with analysing data mining tools
Special SeEn using a white list
Keyboard shortcut SeEn
As a first step, I differentiate SeEn by their content or to use a business term, by the target group:
General search engines with no limitation by topic or region (e.g. Google).
Regional search engines for a special geographical region or language, but without a specialized topic (e.g. Baidu).
Vertical or specialized search engines for a special topic (e.g. SeEn for politic, society & rights information like HuriSearch), document (media) type (e.g. SeEn for images like Picsearch) or target group (e.g. SeEn for kids like Blindekuh).
Updated information SeEn Search Engines with a strong focus on actuality, in fact with a high update rate (e.g. News SeEn like Yahoo! NEWS). These SeEn are often classified as vertical engines, too. But I actually think that these SeEns differ in many ways much from vertical SeEns, so use a separate group for them. First they have a higher update rate than other SeEn, and also they use the kind of information they collect to present the results in a special way. At least there are first vertical offers like Topsy Photos or Twicsy for an image search in social media web sites, science or sports news search opportunities from Yahoo! NEWS (not special mentioned in this collection, as well as) regional news search opportunities too from Yahoo! NEWS (for example for US news). Interstingly, the importance of blogs as an object of a SeEn was not strong enough to lead to vertical products for special groups of interest (at least I have not found any, yet).
News SeEn These SeEn collect information from online newspapers and news portals from the previous 30 days and some like Yahoo! NEWS additionally add purchased news from news agencies. Then they use the special kind of news (like date, source, text parts like headlines and so on) to offer deeper search functionality and result presentation.
Social Media SeEn These SeEn either collect information from weblogs, social networks, microblogging services and other social media web sites (e.g Topsy) or they offer a combination of them (like Socialmention). Like News SeEn, they use the kind of the retrieved content to present special search options or result presentation (for example by date, blogger or tweet author, headline, number of comments etc.). Blog search engines for example use an automatic system for updating the SeEn. If a user publishes a new article the blogsoftware sends a so called ping to the SeEn server to inform about the new content.For more information about news and social media SeEn see: Lewandowski, Dirk: Spezialsuchmaschinen: - In: Handbuch Internet-Suchmaschinen: Nutzerorientierung in Wissenschaft und Praxis. / Dirk Lewandowski (Hrsg.). - Heidelberg: AKA, 2009, S. 53-69. Retrieved from HAW Hamburg - Department Information Web site //Buchkapitel: http://www.bui.haw-hamburg.de/publikationen.html, especially page 67.
People SeEn These Search Engines collects information about people and present the results in a systematic way (e.g. Yasni). Note: Sometimes SeEn for information about prominent people are classified as People SeEn. But these SeEn usually are Special SeEn with a white list of websites like fan pages and they do not offer a systematic result presentation. In this classification they are classified as special interest SeEn (e.g. Celebrities.de).
Local SeEn and Product SeEn are excluded in this overview (see here).
On a second level, I classify the Search Engines by their technical functionality. You will find the categories I use for this below.
Crawler based algorithmic Search Engines: These Search Engines use a Web Crawler, "a computer program that browses the World Wide Web in a methodical, automated manner or in an orderly fashion" (Source) to collect information about websites. They use any kind of algorithm (page rank, number of keywords, ...) to analyze the sites and to display them.
Meta, Multi or Parallel SeEn
Meta SeEn: "A metasearch engine is a search tool that sends user requests to several other search engines and/or databases and aggregates the results into a single list or displays them according to their source. Metasearch engines enable users to enter search criteria once and access several search engines simultaneously. Metasearch engines operate on the premise that the Web is too large for any one search engine to index it all and that more comprehensive search results can be obtained by combining the results from several search engines. This also may save the user from having to use multiple search engines separately." Source
A Multisearch in my mind is a SeEn that offer a single searchbox, and then the user can choose between different search engines (CUSI) or they offer al lot of searchboxes for different SeEn on a single site (One Page Multi Search). The Search themself is always just in a single SeEn - so it is no real Metasearch.
Sometimes SeEn I classify as "Multi" or "Parallel" SeEn are classified as Metasearch too.
Web directories: "A web directory or link directory is a directory on the World Wide Web. It specializes in linking to other web sites and categorizing those links. A web directory is not a search engine and does not display lists of web pages based on keywords; instead, it lists web sites by category and subcategory. Most web directory entries are also not found by web crawlers but by humans. The categorization is usually based on the whole web site rather than one page or a set of keywords, and sites are often limited to inclusion in only a few categories. Web directories often allow site owners to submit their site for inclusion, and have editors review submissions for fitness." Source
These pure Directories are excluded, but sometimes a SeEn is based on a Directory. But in this case it is not the directory entries that are retrieved, they search the collected websites.
Social Search Engines: "Social search or a social search engine is a type of web search that takes into account the Social Graph of the person initiating the search query. When applied to web search this Social-Graph approach to relevance is in contrast to established algorithmic or machine-based approaches where relevance is determined by analyzing the text of each document or the link structure of the documents. Search results produced by social search engine give more visibility to content created or "touched" by users in the Social Graph. Social search takes many forms, ranging from simple shared bookmarks or tagging of content with descriptive labels to more sophisticated approaches that combine human intelligence with computer algorithms. The search experience takes into account varying sources of metadata, such as collaborative discovery of web pages, tags, social ranking, commenting on bookmarks, news, images, videos, knowledge sharing, podcasts and other web pages. Example forms of user input include social bookmarking or direct interaction with the search results such as promoting or demoting results the user feels are more or less relevant to their query." Source
Mobile Web Search Engines: They optimize the surface of a SeEn and the SeEn itself for the requirements of mobile devices.
Fact Search Engines: Present facts instead of websites.
Visual Results Search Engines: "Rather than a simple textual list of results some search engines will provide content in a visual format. This is great if you want a change, or to view results differently. These engines also appeal to students and children." Source
Search Engines with analysing data mining tools: Search Engines which offer its users data mining tools for result analysis. Like Visual results Search Engines, they provide content for example in a visual format.
Search Engines with Blended results: "There are some search engines that will try and blend a variety of results onto one page for you - websites, news, video, images and so on. Good for an overall view of a subject area. Unfortunately there are not very many of them!" Source
Real Time Search: Search Engines primary for news or social media content, which provide results in realtime, so they have a high actuality. For more information see here: Source