Usability is primarily the simplicity and convenience in use of a web- portal (site). Easy navigation is only needed in order to lead to the desired result (i.e. to the required information.)
Useful links:
• http://www.useit.com/ - site of Jakob Nilson, usability expert from the United States
• http://www.usability.ru/ - Russian usability community
• http://www.usability.gov/ - site of the U.S. government about the usability
• http://www.usa.gov/webcontent - site of the U.S. government about web- content
• http://upassoc.org - International Association Professionals' Usability