Usability is a key term that you, as a search engine optimizer, will be listening more of laTely. In essence it means the ability of an entity to complete a function once it is started. How does this apply to SEO? Essentially if you brag that your site promises "SEO guru help" and individuals who log in there do not see any help at all but only affiliate links to other sites with gurus then Google may not see your site as having usability. The more technology progresses it looks the more that marketing types will now be obligated to fulfill the promises that they make with their keywords.
Google, the pioneer when it comes to developing the search engine page ranking algorithms that other search engines use, has now switched its focus to looking at how people find information and what they do when they access a document found in the Google index. Google now has ways of determining how long people look at your web pages, if they like to explore your links to both your archives and outside sites and the relevancy of your documents to any keywords that you have submitted. Google also measures the amount of times your website is put into "My Favorites" in Internet Explorer. The more people that put your website into their favorites, the more Google will like your website and that in turn push it up the Search Engine Results.
Yet another component of usability is your website's functionality. If it does not work, Google and other big sites simply may not bother to list it. As the search engine spiders are constantly crawling millions of sites it could take years for a spider to crawl your site for a second time so you need to get it right before you make your website live!