Web usability is an approach to make web sites easy to use for an end-user, without requiring her (or him) to undergo any specialized training.[citation needed] The user should be able to intuitively relate the actions he needs to perform on the web page, with other interactions he sees in the general domain of life e.g. press of a button leads to some action. („Don’t make me think.“)

Source: http://en.wikipedia.org/wiki/Web_usability

Links:

http://www.useit.com/ (King of Usability)
http://usableweb.com/
http://www.webcredible.co.uk/user-friendly-resources/web-usability/basics.shtml