Accessibility
Essentially, this means having your site accessible to anyone who may wish to view it, regardless of disabilities or impairments, such as impaired vision (including blindness, short-sightedness and colour blindness), inability to use a mouse etc.
It is beyond the scope of this document to discuss how to make a site accessible, but you can read a full description of the generally-adopted guidelines for web site accessibility here:
http://www.w3.org/TR/WAI-WEBCONTENT/
It is considered that search engines are increasingly becoming aware of the need for accessibility within web sites, and are ranking accessible sites more favourably than inaccessible ones.
While the guidelines consider various elements, such as <h1>tags and the like, for search engine optimisation purposes, the most important element is the use of <alt> tags.
<alt> tags are related to graphic images on a page and are visible through Internet Explorer when a site visitor mouses-over the image.
Best practice for both accessibility and search engine purposes is to describe the image, without stuffing the text with keywords.
For example, on the page
http://www.vam.ac.uk/collections/fashion/index.html
the image of the pocket next to the Lucy Locket rhyme, should have <alt> text such as:
History of Fashion - A Pocket
This both explains what the image is and gives the search engine spiders the opportunity to relate the image to the content of the page, which is being targeted for the phrase "history of fashion", in the search engines.