The client and project team spend many months and many dollars to strategize, design, and build a site that fulfills every requirement, satisfies the stakeholders, and drips cool. Then reality rushes back in. Users can't find what they want. Potential customers dump shopping carts without buying. Employees can't figure out the new intranet tools.
Clients try the order entry system once, then go back to the fax machine.
The feedback bin fills with flames. The site was designed to please everyone except the user. The site must take users into account by adhering to high standards of usability. Web site usability is the practice of pleasing the user by making sites easy to use. Fortunately, it is not rocket science.