Search Engine Optimization (SEO) is a key feature to a website's success. There are numerous search engines available; Google, Yahoo, MSN, and AskJeeves are the top contenders. Search Engine algorithms (a.k.a. Search Engine Spiders) are forever evolving but there are basic standards to follow that will always be important when a search engine is indexing your site.

Relevancy

There have been numerous sites in the past that lacked design but had great Search Engine Optimization and are ranked very high for searches. The most important thing to keep in mind is relevancy. A page on your site should not veer off topic and you should repeat the keyword(s) several times within the page.
Example:
Say you have a page about family reunions. You would obviously want to entitle the page Family Reunions and within your first few paragraphs you should also use these words at least a couple of times because the first 250 words on a page get better relevancy.