Web accessibility, as the name suggests, is a measure of how accessible a website is to an individual user with the equipment they are using to view the website. Traditionally, accessible design has tended to focus on users with disabilities, for instance, visual impairment. There are recognised standards of compliance, but, as ever there is a degree of subjectivity in judgement and there can be some ambiguity in interpretation.
A lot of people question whether this is something they need to worry about; "our users are 'average' users". Whilst the logic of this sort of statement is obviously highly dubious, there are a whole of host commerical reasons that companies should ensure their sites are accessible. For instance, do your customers use mobile devices for browsing your website?
The usability of a website is a far more subjective assessment. In brief, it's a measure of how usable a website is from the perspective of the end user. As websites do so many different things, it's difficult to come up with a single catch-all definition - it's more to do with an approach. You could describe it as the effectiveness of a website to deliver information.
Visit the full gallery here: www.flickr.com/hello7dots