There is an incredibly strong link between Usability and SEO. First, you have to start with the knowledge that the business of the search engines is enhanced by having the most relevant results in their index (the ones that do in fact answer the user's question quickly), and this means that usability matters to them. Therefore it is in their strategic interest to develop an understanding of a site's usability.
There are many ways that the search engines can collect basic usability data. For example, do users bookmark your site at major sites such as del.icio.us? Do you have a high bounce rate (people who view only one page, or who don't stay long on the site)? These are just a couple of basic things that a search engine can look at to measure usability.
Second, usabilty is a key factor in driving the acquisition of high value links. Trying to get a major university or a government site to link to yours? What do they see when they come to the page you are trying to get them to link to? Do they understand it right away? Or does it confuse them?
At an architectural level, having a clean site hierarchy and an easily understood navigation structure also benefits both usability and SEO. This means things like a logically thought through hierarchy that matches up with the nature of the content you are providing in an easy to understand way. It means having a consistent global navigation structure, and a breadcrumb bar.
There are many great resources on usability. For example, there is Jakob Nielsen's usetit.com and Kim Krause Berg's Cre8PC that delve into the specifics of good usability then I will attempt to do here.
What I want to emphasize here is one key point: Usability comes first, and SEO comes second. Don't get me wrong, I am not short selling SEO here. I think it's incredibly important (well, OK, it's a key part of how I make my living). But when you are looking at a new site design, or are re-evaluating an existing site, you need to start with some basic questions. Here are some examples:
- What is the purpose of the site? There are many good answers to this, such as generate leads, sell products, reduce support costs, or sell advertising. Understanding the answer to this question is where it all starts. Fulfilling this purpose can be thought of as a conversion.
- What types of visitors do you want to attract to the site? Who are the people you can bring to your site that might convert, either in the short term or the long term? For many sites, there are many distinct groups of visitors. They can be divided into groups such as repeat visitors and first time visitors, visitors from different geographies, shoppers and researchers, etc.
- For each group of visitors, think about the types of information and products and services they want to see on your site. What would make it useful to them? What are their goals, and how can you help them meet them?
- For each group of visitors, now that you know what they are looking for, what will their experience be like when they arrive at your site? Will they know where to find what they are looking for? Or will it they arrive at your site, be one click away from converting, and simply leave because they don't see what they are looking for?
These are some of the most basic questions that every site owner needs to consider. Advanced companies do usability design and analysis, including live usablity testing with real users on their sites. Some companies get more sophisticated still, and incorporate the use of eye tracking gear, to really get down to the nitty gritty details of how people see their web pages.
While you may not be in a position to take your pursuit of usability quite that far (although you should if you can), you need to be thinking about it. Getting tons of search engine visitors and a low rate of conversion will not help you much. In addition, if the search engines have their way, a low rate of conversions (then sites in comparable markets) will also lead them to lower the amount of traffic they send you.