Usability is the attribute of how easy something is to use. The term 'usability' can also refer to the methods used during design to ensure 'ease-of-use'.
Usability can make a massive difference to the perfomance of a website, and as a result investment in it can have amazingly high ROI. Similarly, a lack of usability will result in people leaving your site.
Usability Guru Jakob Nielson believes that key factors of Uasbility are:
To increase usability you need to consider the design and ask questions that will move those factors in the right direction.
The easiest way to make sure you're going in the right direction is to conduct usability studies. Getting the users to imput to the design.
I offer both audits and consultancy on usability. I can help in a variety of situations from identifying key areas of improvement, through to developing strategys for User Centred Design. Please contact me if you would like more information.