Usability.gov is a good resource for web usability.
Web Usability is all about efficiency. Does your design and functions help the sites ultimate goal?
Some basic rules are:
- Ease of use of the sites information architecture.
- Present information in a clear and concise way.
- Make navigation obvious, the path you want the user to follow.
- Limit likelihood of user errors on the site, make errors limit easily fixed.
- Put the most important element in the right place.
