1. 搜索引擎：
顾名思义，搜索引擎基本上被定义为旨在执行网络搜索并使用户能够在 WWW 上定位信息的应用程序。搜索引擎是机器驱动的，用于查找分配给单个词的资源，例如网页、usenet 论坛、视频等。
示例：雅虎搜索、ASN、实时搜索、谷歌搜索等。
2. 主题目录：
顾名思义，主题目录基本上被定义为包括有组织的可浏览主题类别和子类别的网站集合。目录是人为驱动的，因此有能力提供更高质量的内容。它们被认为最适合浏览和搜索更一般的性质，并且可能包括用于搜索他们自己的数据库的搜索引擎。
示例： Yahoo 目录、Open 目录、Google 目录等。
搜索引擎和主题目录的区别：
|
Search Engine |
Subject Directory |
|---|---|
| It is an online tool that is used to locate or search information on WWW. | It is an online database of websites and online information set up by subject and category. |
| It is generally maintained and reviewed by computer robotics. | It is created, maintained and reviewed by human editors or experts. |
| It allows users to find or locate information on internet using phrases and keywords. | It allows users to find or locate information in internet using hierarchy. |
| It locates and display relevant information based on formula and algorithm using automated software program. | Directories are organized by human beings who discover new websites and relevant information by exploring internet themselves and by reviewing submitted websites. |
| They are used mainly for research, shopping and entertainment. | They are used mainly to search broad topics, current events, organization or business homepages, etc. |
| They are bigger than directories as it includes billions of web pages. | They tend to be smaller than search engine databases. |
| It is considered good to look for specific or complex information and its advanced features allow to refine search and improve results. | It is considered best if one wants to browse broad topics and it can give good idea about amount and type of web-based information on topic. |
| User search content or information on database of search engine. | User search only websites included and identified by directory. |