There is often confusion with this term as it is taken to mean meta name="Keywords" these serve no purpose as search engines have disregarded them years ago.
Keywords in this instance i mean on page
Keywords are the most important in this aspect regarding Search Engines,It is what your site is about, emphasis on the purpose of your site.
Search Engines reportedly use an algorithm to determine the "Keyword Density"
Total Words ÷ Keywords= Keyword Density
Choose keywords that best relate your information, products or services.
It is relatively easy to rank high for terms that in reality are not used by genuine web surfers, there fore have little real competition for that term.
This is often the ploy of SEO experts to impress you with rank position , however if real potential customers would never use that term, is serves at best little purpose.
This should include phrases that may be used as most searchers do not rely on one word to search. it can also be of use to localise searches. IE baker is a very wide search item while "Baker in Doughville Yeast County" will help to narrow the field
The most important thing to remember is that the content of each page is different, so only use keywords pertaining to that page.
When you name a new page include a keyword relevant to that pages content
These are used by search engines to desribe your site, while each page should have a unique relevant description of it's page. the first page is most likely to be your indeex page so this should be a considered description of your site
you can add keywords to page title make each title page unique and relevant to that page
Alt tags can contain keywords, but again should be a description of your image.
Content is King. recomended size of at least 350 words per page, Remember the formula for keyword density.
Be thoughtful of how you word your content and incorporate your keywords. do not however make your content unatural by repeating keywords with little logical reason, content should always seem, and be naturally constructed language.
Do not over stuff your page with keywords as you could be penalised
Hyperlinks are text links to other pages on your site. links so the Search Engine will have a relevant text link to follow to that page
Once your site is up and running, spend time developing links back to your site from other websites.
Make the link text relevant to your site. Quality links come from established sites relevant to your site ie a similar industry or supporting industry/ service to yours.
Writing articles including links back to your site can be of use especially where you can link to pages other than your index page.
this is called deep linking and should also be considered carefully when building all your back links.
It was a practice to build links by submitting to directories.
There is still some merit to this but its importance is diminishing, Google for example disregarded results from many these.
There is still a growing market of earning income from direcories including buying old sites of high Google page rank, and selling links before google re asses the site and drops its old ranking. i suspect with googles updating to avoid its un natural attempts to aid your site this will continue to diminish.
However some carefully built links, in correct directories, in relevant categories could still yield some juice to your site