Once you have established your keywords for which you have to optimize your site for,you have to think about how to get ranking on those keywords.For that purpose you have to create keyword rich pages(or in short KRPs) in which a particular keyword is repeated n no of times so that it gets the top rankings in the major search engines.
This is the first and the most important tag in which you should insert the most important keyword or keywords(but there is a limit) you have to target.This tag is displayed in all the major search engines.
The Description Tag is used by many search engines to provide a short description of the page that is displayed in the search engine so it should be keyword rich.The content of this tag is slightly longer than the title tag.This tag is also very important from search engine point of view.
The tag is becoming less and less important as far as search engines are concerned.Yet it is included to be on the safer side,in this tag keywords are inserted according to their importance,a particular keyword is not repeated more than 3 times and the same keyword is not inserted one after the other.