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
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