Simple.Just check out the keywords that shows the category of website, in other words those world around what website is based. That call keywords. They work like blood in body of website.
Keywords are those words which you tag on your site and if anyone search using those words, your site might be seen on the SERP. Except those keywords, rest of the words are ordinary words.
wiseman is right that you can meta tag your keywords to get better results, but that is not the only thing you need to do. You need to get a decent keyword density in the body as well, i attempt for 5-7%. If you are unsure what your density is then you can run it through a density cloud at webconfs.com. This will give you a list of all the keywords that apply to your site and how often they occur. You do not want to have any keyword over 10% or you will get penalized by the search engines.