Definition - What does Search Engine mean?
Techopedia explains Search Engine
To simplify, think of a search engine as two components. First a spider/web crawler trolls the web for content that is added to the search engine's index. Then, when a user queries a search engine, relevant results are returned based on the search engine's algorithm. Early search engines were based largely on page content, but as websites learned to game the system, algorithms have become much more complex and search results returned can be based on literally hundreds of variables.
There used to be a significant number of search engines with significant market share. Currently, Google and Microsoft's Bing control the vast majority of the market. (While Yahoo generates many queries, their back-end search technology is outsourced to Microsoft.)