Search engines, on indexing websites, look at where the individual words and phrases occur. Words in Title tags, headings (h1, h2 etc) and emphasised in bold, italic or capitals are noted. You should look at your website’s content in the way your website visitors do. If a word or phrase is emphasised then you are clearly wanting your visitors to notice it but if you had every word in capitals, bold etc, then you would not be emphasising anything would you, plus your website would look a mess!
Google has had many years to develop algorithms to work out relevance and importance and in addition, their studies of language use mean that they can determine whether the content makes sense and what words essentially mean the same as others ie search engines have a built-in thesaurus. This is why in search results you may find websites which do not mention the particular word or phrase you placed in the Google search box BUT do relate to the same topic.
By having studied language usage over many years, Google is able to tell if your web page makes sense or is just stuffed full of keywords or whether it is just a list of links to other websites. The algorithm is also able to tell if the content of your website is copied from another website. Google is wise to the many methods used to try and manipulate search results and consequently if you use any of these bad practices you risk having your website removed from the index completely.
If you focus on providing honest, useful content that is not plagiarised and is updated regularly then you can be sure that your visitors will keep coming back for more – they will bookmark you, subscribe to your RSS feeds and essentially vote for your content – AND if your content is interesting enough to keep visitors coming back for more then the search engines will too. Interesting, changing original content will keep everyone happy and is the best SEO you can have. Easy ways to have useful changing content on your website is by having a Blog, a News page, FAQs all of which you keep regularly updated.