Usability is the term used to define how easy a user interface is. It can also refer to the different ways interfaces can be improved during the design stage.
There are five components that are used to define usability:
Ease of learning: How easily do users finish the tasks when they first come across the design? For example, if you run a website that requires user sign-up, how easy is it for users to access and complete the sign-up form?
Efficiency: How quickly can users complete the tasks, once they have learnt how to use the system? In the case of the example we previously used, this could refer to navigating between site pages or completing a user profile.
Memorability: How easy is it for users coming back to the system to re-learn how to use it? When returning users access the system after a few weeks or months away, they should not feel as though they have to start learning everything from the top.
Errors: What kinds of errors, and how many of them, do users make? How do they recover from making those errors? Errors include trouble accessing pages or using features.
Satisfaction: Is the design pleasant to use? Most users will not return to a website that is unpleasant or difficult to use.
Usability is extremely important because people will leave (never to return) if you website is not easy and simple to use. If users have to constantly go back to the home page to find another page, if they cannot navigate easily, if it is difficult to subscribe or unsubscribe, they will not use your website.
It is easyto figure out the usability for your website by conducting testing, but you need not test it on dozens of people. Ask a few people (3-5) to complete 3-5 tasks on your site. Look at how long it takes them to finish these tasks, the errors they make, and where they get stuck. Take notes, and ask them to describe their experience to you, so you can make the required changes.