What is the difference between Usability and Accessibility with regards to web design

Usability: making a website as simple and fast to navigate as possible, and presenting information / products in an easily readable format

If you are selling something on the web prospective customers don't want to spend more time then they have to find what they are looking for, make it simple and easy.

Accessibility: making the content of a website available to everyone, including those with less-than-perfect eyesight.

If you come apon a site that you can't see well or is cluttered with unreadable content people won't stay any longer on your site as the next click of the mouse.