Keyword Research - SEO STARTS HERE...SERIOUSLY
Above all else, make sure the keywords the page is targetting are actually used in a keyword search.  It seems silly to point that out, but you would be surprised at how many small and medium sized business owners have no idea what words people use in a keyword search to find their product or service...or worse, they assume they do know and they're WRONG. 

Do you know which words people use most in keyword searches for a given industry?  Are you sure?  In the spirit of "better safe than sorry", use a keyword suggestion tool to make sure the terms the site targets are actually the terms people use in a keyword search.  A site could be perfectly optmized for any given term, but if that term is never used in a keyword search, what's the point?  

Nothing is more important to On-Page SEO than keyword research.  If you're not willing to spend the next 3 months optimizing a website for one set of terms, only to find out they are rarely if ever used, do some research.   Having a #1 ranking doesn't mean anything if it's for a term no one searches for..."clueless numpties".

Even if you're certain of the most popular terms for your industry, it is a good idea to use a keyword suggestion tool before ever beginning the process of Search Engine Optimization.  Rather than choosing just one term, like "photographer", using a keyword suggestion tool may help you optimize the site for multiple related terms that you may not have considered (or mistakenly believed no one used in a keyword search) - like "portrait photographer", "family portraits" and "senior pictures".  That will not only reduce the competition for the more general term "photographer", but it will also help you target those individuals who are looking for a specific product or service, like "senior pictures".

Keywords vs. the Keywords META Tag - What's the Difference?
As illustrated above, keywords are the terms a person would use in a search with a search engine, like Google - i.e. "caterer jacksonville fl".  It is important to understand that a keywords META tag should contain the most important words for a site, but the tag itself no longer has any impact on a site's ranking in search engine results. 

A keywords META tag is HTML code that was previously used by search engines to identify a site's keywords.  Search engines no longer rely on this information.  Simply adding a word (or words) to a keywords META tag will not cause the site come up in the search results for that term.  Today, keywords META tags are used by people who review the HTML of a given site, but have no impact on the site's search engine ranking.