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1. Crawler-based Search Engines
- Rely on "software spiders"
- To crawl and index web pages.
- Automatic.
- Example: Google, MSN, Yahoo web search.
2. Human-Powered Directories / Categories
- Rely on submissions from users and web site owner to populate their indexes
- To crawl and index web pages.
- Example: Yahoo Directory, Open Directory
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3. Pay Per Click Engines / PPC Engines
- Web site owners bid on keywords
- Whenever a visitor clicks on a keyword ad in the search engine results
- Web site owner pays the bid amount
- Example: Overture, Baidu, Looksmart
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