Your website has to be many different things. You need it to be optimized for search engines, which means it has to be crawlable, indexible, keyword rich and PageRank high. You have to provide content that the search engines recognize as relevant, and learn how to ensure a speedy evaluation and approval. You want to be listed on the SERPs in a favorable position, so you have to sculpt your pages to spread your PageRank around. You have to decide when to no-follow and no-index, where to get your links from and how.
All of these things can be described as SEO. Many of them are also site architecture – your links, both internal and external, your flow of traffic through your website from landing page to final checkout, etc. There is another facet that is just as important, and your SEO efforts have to reflect this need as well.
You need information architecture on your site as well; in your content, through your links, and in the way that you set up the traffic flow. Information architecture is what makes visitor stay on your site once they arrive. You have to optimize it for the user, making it easy for them to find and utilize the information on your site! If the user arrives on your site to be faced with a beautifully laid out town with no map, he or she is going to become quickly disillusioned.
Usability and findability are key to information servers like most websites; even strictly sales sites have to provide information and this is what the visitor is generally looking for. Providing a site that is solely put together for search engines will lose you customers every time. With the right attitude, you can provide a great user experience without sacrificing any search engine worthiness; just remember that your site architecture and your information architecture have to walk hand in hand.
SEOs need to pay attention to both. We can’t ignore the need for user friendliness. Ours is no longer a simply technical job – use tool A with component B so it will do C; we have to make the handle on tool A fit the hand of user D, so the action of C is enjoyable and results in user satisfaction.