When people talk about search engine ranking, its likely there talking about two subtetly differant things.

First, is your ranking for a specific keyword. Where your pages are placed when a user carries out search on the major search engines.

Are you on the first page? in the top 3 sites?, or is your site buried on page 10. Its very difficult to get natural traffic from search engines if the pages within your site are buried so far back in the SERPs.

Then you have PageRank which is Googles trademarked PageRank technology, or what its often called - The holy grail.

PR is a numerical representation of how important a page is in relation to its target audience. PR is primarily determined by the quality of inbound links to a page, and the traffic that results from those links. Having a page with PR7 link to your site is better than a page with PR3, but quantity is also factored into the equation.

Over at webworkshop.net you'll find an excellent and very thorough explanation of PageRank.

In the ranking tools section, you'll find various tools for retrieving the pagerank of pages.

In order to increase your search engine ranking over time, the best thing you can possibly do is add more interesting and relevant content as often as possible, and promote your site

Promotion doesnt have to involve big budget PPC campaigns, its possible to have a PR7 site that never spent a cent on advertising, with promotion through like minded sites (link exchange), forums, and blogs etc.

In the end though its quality content that will secure yours sites ranking.