What is the difference between Usability and Accessibility with regards to web design and why are they both important?

Usability is the quality that refers to the speed with which users learn to use something, the efficiency they attain while making use of it, how easy it is for them to remember how to use it, how error-prone it is and the level of satisfaction that they attain from using it.  In another words, usability is a measure of how easy it is to use a site.  While Accessibility is a feature through which people with disabilities can perceive, understand, navigate, and interact with the web.  Web accessibility includes all types of disabilities that impact access to the web and includes: visual, auditory, physical, cognitive and neurological disabilities. 

Both Usability and Accessibility are important because they are ways in which all individuals interact with the web, and a site is not usable unless it is accessible to the masses.