Search engines work largely by matching the words in the search query, with the words on the web site. Pages that match highly to the search query words will tend to rank highly in the result.
Search engine tweaks, and being true to your material
There is a tradeoff to consider between being true to the material you are presenting, and twisting it to match the ranking algorithms in the search engines. That is, you can finagle your web site to get a high ranking in the search engines, but in doing so do you lose site of objectivity and accuracy?
For example, if your passion is writing about digital cameras you have two choices in creating content for your site. One choice is to write about whatever you want, no matter where the writing takes you. That would be an honest expression of artistic creativity. But, what if the result didn't rank very well with the search engines?
The typical advice to increase your ranking with the search engines is to use lots of the precise words and phrases used by people in search engine queries. Therefore you need to study what the marketplace is looking for, and then shape your web site to match whatever are the keywords most frequently used. In a way this is the same as any commercial writing, because they are always wishing to provide books or whatnot that people will buy. The complaint is, though, that this is a McDonalds-izing of everything, that in search of large sales, every item in the marketplace is homogenized to remove possibilities to insult or confuse, but in the end results in removing any uniqueness and making everything look like everything else.
In this digital cameras example, you might find yourself writing about some particular camera you hate, just because lots of people are looking for information about it.
Key words
There is a lot of arcane science that goes into the order search engines return their results. Obviously it's the order results are given which often determines which site will be chosen. (hint: it's the first few) So there's a competition among web site authors to get into the first few listings of any search.
One aspect is to predicting what search terms people will use to look for the information you're offering. No matter the topic, there are a range of words and phrases used in that topic area.
For the purpose of writing your web site content, so long as you stay to the topic and do a diligent job of writing about that topic, you will automatically use the variety of key words that people will use to search for the information you're offering.
It is the words someone types into a search engine which are the "key words" or "keywords" that you need to pay attention to. If your site is rich in the keywords for a given topic, then it seems your site will be more highly ranked than other sites. For those keywords.
It has to do with relevance. If your web site is more relevant to the keywords used in someone's search, then your site will be listed before other sites. Often.
For example in the resources page I list Word Tracker. With this site you can see exactly which keywords are being used by real live people around the Internet. It shows the keywords, related keywords, and the relative popularity. With this information you can gear your web site to contain those keywords. By having content on your site relavent to common searches, your site should rank higher than others.