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Search engine is a generic term used in describing human-powered directories and crawler-based search engines.  Listing gathering is radically different. 

In crawler-based search engines, like Google, listings are automatically created.   People browse through what was located. 
When web pages are modified, they are automatically located and affects the listing.   Titles of the page, copies of the body and all otherPage elements have a role to play. 

On the other hand, a directory powered by humans are dependent on man for the listing.    A short description of the whole site is submitted to the directory or the editor creates one for reviewed sites.  A search locates for words that match the description submitted. 

The listings are not affected by modifications in the  web pages. Beneficial things in the search engine improvement are useless with directory listing improvement.   A good site containing good content being likely to be reviewed free compared to a poor site is the only exception. 

During the early years of the web, a search engine used to be presented with results from crawler-based engines or from listings of human-powered engines.   Presentation for both results are commonly presented.   Usually, one type of listing is favored over another type of listing by a hybrid search engine.

There are three major parts of a crawler-based search engines .  The spider visits a web page, interprets , and then follows links to other pages within the site.  The site is regularly revisited by the spider to search for modifications.
Everything the spider locates is incorporated into the second element of the search engine, the index. The index, sometimes called the catalog, resembles a giant book which contains copies of ever web page located by the spider and updated with fresh information. 

Sometimes, finding and adding modifications to the index of the new pages may take some time.   Until such time information is added to the index, they are still not available to the search engine.

The third part of the search engine is the search engine software. Finding search matches, recording them in the index and recording them in the order of relevance after browsing through millions of pages is the responsibility of this program. 

Although most crawler-based search engines contained the basic parts mentioned, the difference lies in the manner of tuning these parts.   This is why different results are produced by different search engines.