What are keywords and how to choose them?

Hi friends, do you know keywords plays very important role in improving one's google adsense earning. I am creating this forum to discuss google adsense earning improvement methods. First and very important way is the proper use of keyword. Keywords are the specific set of words which people usually use to search.
How to choose proper keyword?
This is the basic question which will arise in your mind that how we can choose proper keywords? So friends before writing any article just ask your mind the question that if you have to search google about that article then what you will type as your search text and that search text can play the role of keyword. After this try to involve that keywords is your article and also try to put that keyword under bold code. If you want further clarification then please ask by comments.