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. Accessibility has unique concerns, but
there is considerable overlap between the two. What makes a website more accessible often makes it more usable.
Usability is a frame of mind. It's not a one-time effort. It's vital to stay focused on usability 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. 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 professional. The latter is best if you lack the time, knowledge,
or interest.
Whether you do it yourself or get some guidance, usability is a valuable long-term investment.
