The second type of search engine is the index. Some examples a Web searcher may be familiar with are Google and AltaVista (links open new windows). Index search engines allow the searcher to input either a simple search—just type a few words into the main search box and hit "Go"—or link deeper into an advanced search engine and perform more complex searches. These Websites often allow Boolean searching and phrase searching.
A subset of this kind of search engine is the natural language search site, such as Ask.com (new window), which allows you to formulate your search in the form of a question (i.e., What is the current treatment for hives?) and arrive at a list of Websites that hopefully will answer the question. The disadvantage of this type of search is that the searcher is at the mercy of their own vocabulary and search skills. There often is no directory structure to help suggest topics or alternative wordings.