Web usability is a broad term. It encompasses HCI (Human Computer Interaction), information architecture, and usability testing,
among other things. Accessibility is also an integral part of usability. It has unique concerns, but
there is considerable overlap between the two. What makes a site more accessible often makes it more usable for everyone.
Usability is a frame of mind. It is not a one-time effort. It's vital to stay focused on usability throughout throughout the entire lifecycle of your site, product or service.
How well a website prospers (or how quickly it perishes) may be influenced by its usability.
If a site is easy to use and accomplishes users' goals, it will more likely become popular and draw repeat visitors. Usability makes so much business sense, it should be common sense.
Many usability resources are available online and in print. You may start with those,
or find a firm to conduct your usability research. The latter is best if you lack the time, knowledge,
There is an ever-growing number of knowledgeable usability firms. Whether you conduct
usability research yourself, "in-house" or with external assistance, the information
you uncover will be an invaluable long-term investment.