Keywords are words or phrases that describe content. They can be used as metadata to describe images, text documents, database records, and Web pages. A user may "tag" pictures or text files with keywords that are relevant to their content. Later on, these files may be searched using keywords, which can make finding files much easier. For example, a photographer may use a program like Extensis Portfolio or Apple iPhoto to tag his nature photos with words such as "nature," "trees," "flowers," "landscape," etc. By tagging the photos, he can later locate all the pictures of flowers by simply searching for the "flowers" keyword.
Keywords are used on the Web in two different ways: 1) as search terms for search engines, and 2) words that identify the content of the website.
1) Search Engine Search Terms
Whenever you search for something using a search engine, you type keywords that tell the search engine what to search for. For example, if you are searching for used cars, you may enter "used cars" as your keywords. The search engine will then return Web pages with content relevant to your search terms. The more specific keywords you use, the more specific (and useful) the results will be. Therefore, if you are searching for a specific used car, you may enter something like "black Honda Accord used car" to get more accurate results.
Many search engines also support boolean operators that can be used along with keywords to further refine the search. For example, you may search for "Apple AND computers NOT fruit" if you only want results related to Apple products and not the kind of apples that grow on trees.
2) Web Page Description Terms
Keywords can also describe the content of a Web page using the keyword meta tag. This tag is placed in the <head> section of a page's HTML and contains words that describe the content of the Web page. The purpose of the keywords meta tag is to help search engines identify and organize Web pages, like in the photos example above. However, because webmasters have been known to use inaccurate tags to get higher search engine ranking, many search engines now give little to no weight to the keywords meta tag when indexing pages.