Each search engine and directory has its own set of Web page ranking and listing criteria. Generally, search engines use proprietary ranking algorithms to determine the quality and relevance of a Web page. The search engine will then decide whether or not to add the page to its index.
When calculating page rankings, search engines focus on Web page copy, Title tags, keyword usage, and the amount and relevance of inbound hyperlinks.
Most search engines use spidering to collect the listings for their indices. Web directories generally rely on human editors. The editors decide if a suggested URL will be included in the directory, and if so, which category and subcategory. Most directories present their listings alphabetically.