tedster - 1:43 am on Aug 15, 2012 (gmt 0)
There's the copy you write on the page itself.
There's the internal links that point to the page from other pages on your site.
There's the topic of pages around the web that mention or link to your website.
There's the link text on pages around the web that link to your website.
There's the way people discuss your site in various social media.
I'm sure there is a lot more that search engines can use - but the key is to actually BE about the topic those keywords point to.
In the early days of search engines, they weren't very good at all when they used pure "text match". Today, they use a lot more signals, including other words on the page that are not even directly the same keywords. For example, using a phrase like "medical check-up" on a page about doctors would reinforce the main keyword. It's a pretty deep science at this point.