What is the difference between Usability and Accessibility of a website?

Usability focuses on how easy and quickly for everyone it is to use the website.
There are several factors:

1. How easy it is to learn the use the website
2. How quickly you can perform a certain procedure; such as finding a certain information, using the navigation bar, or downloading material.
3. How simple it is to remember on how to use certain tasks
4. Making sure you inform the user on how to use the website, before he makes a mistake
5. Making sure the user is satisfied on how he maneuvers around the website

Accessibility focuses on ensuring that everyone can access the website regardless of disability or technology.

1. People with mobile phones and different monitors 
2. People with slow connections (not everyone has broadband)
3. People with Disabilities, for example: Blind people who uses a screen reader, or deaf people who rely on sound