When you look for information online, where do you turn first? For many people, the answer is a search engine such as Google, Bing, Yahoo or AOL Search.

Essentially, a search engine is a digital index. Like an index in a book, the search index takes available content and arranges it by topic and sub-topic. Each search engine compiles its index using its own frequently refined methods. Most indexes start with an automated program known as a spider or crawler that visits sites in a network (such as the internet) and uses keywords or phrases on each page to classify the information it finds. Once it has completed indexing, the spider uses links to other sites to find more new content to index.

The index for a printed book is alphabetically searchable. The reader turns to the back of the book, selects the appropriate topic from the list and uses the provided page numbers to find the reference. Search engines' indexes contain millions of pages, so alphabetical searching is impractical; instead, they rely on a different standard -- query language -- to help users find what they're looking for.

A search engine’s unique strength lies in its ability to "learn" based on how searchers interact with the data it presents them, thereby allowing it to provide an additional element of context when an user searches for an ambiguous term. For example, is someone who searches for "eagles" looking for information about the birds, the football team or the band? The answer might depend on factors such as the season or the searcher's geographic area -- to which the search engine can adjust accordingly.