As soon as someone mentions the word “keywords” everyone associates them with their websites.  Which is great, because that is where you would use them, not to mention in advertising campaigns as a whole.  Your SEO campaigns start with keyword research and what people are using to get to your site, basically what they put into the search engine as a search query to get results generated for them.
After you get the keywords generated, you then should be starting to incorporate them within your site, in the Title, Content, anchor-text, you then can put them in your meta tags, aka meta-keywords.  This is the back end source code of your site that pretty much labels your site on how search engines should categorize it.  A very crucial part of the business but you have to keep in mind, putting down every single word in your content in the keywords is not going to get you found that way.  Careful choosing and experience will get you picking the correct terms, and remember even if you are listed at #1 you still may not be receiving traffic.

You can use the Google Keyword Tool, Keyword Discovery Tool or WordTracker for information on keywords if you cant decide what works.  Although this is half the battle, it is still a chunk of what needs to be done for the site, wash the keywords or take out duplicates and any other terms that may not work for the particular content of the site and than redo the process until you have about 7 unique keywords per page.  Yes per page, otherwise you will be competing with your own pages and cause a bit of an issue when attempting to rank all your pages.