The history of keywords in the Web's context is as old as the Web itself. The first search engines were little more than keyword finding machines. The reputation of the keyword as a concept is somewhat polarized. On the negative side, there are concepts such as keyword stuffing and search engine spamming where aggressive use of keywords is one factor. On the positive side is the importance of keywords and key phrases in PPC and textual ad management that is a key factor in a billion dollar industry.
The source of keyword and key phrase discovery is either on-site or off-site. Dynamic Web sites with user contributed content change frequently and so does the set of keywords that are the basis of the Web sites' reasoning and understanding.
Most Web sites that rely on income of some sort to stay alive do their business in a fierce competitive environment. Off-site keyword and key phrase discovery can help them to analyze the competitors and discover new areas of user interest on the Web. It also helps in identifying the keywords and key phrases to use for PPC and keyword-based advertising campaigns.
There are many tools available for webmasters to perform keyword and key phrase discovery. Some are free or free to try. Below is a list of interesting tools.
Keyword Cloud - Analyses keywords on a Web page and displays a cloud with keywords sized according to density on page.