In the early days of the web, people realised that using most sites was difficult, and applied the term 'usability' to describe how easy a web interface is to use. A web usability testing industry emerged, which has taught us a lot about where things go wrong by watching real people using web sites and comparing the ease-of-use of different design solutions.
There has been a long-running "design/usability" debate in the web industry, with different people arguing that one is more important than the other. This argument is based on a false idea that design and usability are opposing forces. They aren't.
Separating usability from design is no longer helpful. The problem is a design problem.
When designing products that have a communication function, usable design is simply better design, because it makes a product better at its job. Usability is a central element to successful design. There is no either/or.
While it's certainly useful to test how successful a web site is, there's little point in discovering that your design doesn't work after you've built your product.
Testing can help us work out which ideas work better, but it can not give us new, creative solutions to design problems, or tell us how well a site achieves its non-functional goals. Excellence can only be achieved by building usefulness into the entire design process, and applying user-testing at key points to help polish the finer detail.
While I've been described as someone working in the usability industry, I don't do usability engineering. What I do is design. I'm passionate about ease-of-use, and it's central to the work I do.
When I use the term "usability" on this site, I mean design for usability - designing to make stuff easier to use.
This is the one must-read usability book for all web designer and developers.
Steve Krug's masterpice (now in its second edition) achieves the amazing feat of making very complex concepts simple enough for anyone to understand.
It's a very easy, lighthearted and quick read, thanks to great writing, smart page design and lots of pictures.
The second edition has several worthwhile additions over the classic original book, with 3 new chapters covering important stuff like accessibility.