Usability is the measure of how easily something can be used (in the manner in which it was intended). In a nutshell, it considers:
-who are the users?
-what do they want to do?
-in what context is the user working?
-what should be left to the user?
-what should be left to the machine?
-can the user easily do the intended task?
-is documentation available?
-can the user recover from errors?
Once the aforementioned questions have been answered, you can apply the following basic principles to web pages you develop:
-let the users know where they are
-tell the users
where they can go
-divide your pages into clearly defined areas
-make it obvious what is clickable
How easy is this to use?