They retrieve documents, extracting information and indexing it, and storing the index in a database. Retrieval of documents is by means of hypertext links from already retrieved documents and crawling of newly submitted sites.
Directories tend to be compiled by human endeavor.
Know the difference. Generally, search engines have difficulty following both frame links and image maps. This information will help indicate how comprehensive the index is in covering a site and its content. Also, the performance of a search engine, in terms of recall and precision, relies heavily on its indexing strategy. Crucial here is what information is extracted from each document.