From Wikipedia
Web usability is the ease of use of a web site. Some broad goals of usability are the presentation of information and choices in a clear and concise way, a lack of ambiguity and the placement of important items in appropriate areas. Another big concern for usability is that the website is appropriate for all ages and genders.
Usability needs to be a prime factor for consideration in any web development project. Generally, if the usability from the perspective of a website user hasn't been thought through at the design stage, the end result may well be confusing, difficult to navigate and, ultimately, unappealing. Blending great site design with usability is an essential part of website building and one that we take seriously.