The perfect keyword is somewhere in between – very relevant to your site and also a popular search query. How do you know when you have chosen the right keyword to use for optimizing on your site? You don’t. You’ll always have to wonder what the opportunity cost is for ranking for one word instead of a different word. It’s my opinion that you shouldn’t focus on any more than 3 keywords per page to try and rank. When you get past three you’re spreading your page thin. When doing keyword research I think its important to remember the end goal is more customers, not just more visitors. It makes more sense to me to have more pages that rank well for very relevant keywords rather than a couple pages that rank well for popular keywords.
Keyword research is finding keywords with the right balance between relevance and popularity to use towards getting your web page more targeted visitors. Keywords that are very specific and relevant tend to have less competition and therefore, are easier to rank higher for. Broad and generic keywords are searched for more often and therefore, are more popular and harder to rank higher for.