I've helped a lot of people improve their websites.  There are different kinds of work that people ask for, like changing the layout of a site, updating the content, or custom programming.  By far the most important aspect of the site is its keywords!

The presence and choice of keywords is the single most important factor in the success of a website.

Take a second to re-read that last sentence.  Most people (and designers for that matter) don't give enough consideration to keywords when building a website.  Keywords determine how Google sees your website.  Google is the king of search engines... just take a look at their stock price :D

Here's the basics of how it works.  When you make a webpage, Google reads the webpage and saves all of the important phrases (i.e. keywords) on the page.  Google does this for almost every page on the internet.  When someone enters a search term into google.com, Google tries to match the search term with the keywords it saved from each and every webpage.  Webpages that have keywords that best match the search term will be displayed at the top of the Google results page.  Pages at the top of the results get way more visitors.  For a business website, the first step to making money is getting visitors.

Four Simple Do's and Don'ts of Choosing Keywords:

1. Do choose keyword phrases.  A phrase has multiple words.  It is much easier to rank for "used cars in Worcester" than "cars".  This has the added advantage of making your website very specific to a small audience.  This is a good thing.
2. Don't make every other word a keyword.  Focus on 1-2 keywords phrases per page.  Too many keywords on a page will water down the page (Google won't know which words are most important on the page)
3. Do use the Google Keyword Tool to get ideas for keywords and see which keywords Google found for your site.
4. Don't assume your keywords will generate traffic or conversions.  Test your keywords if you can.  It's likely that half your keywords will be worthless. A pay per click campaign is good for this.