Usability is critical.
If a website is difficult to use, people leave. If the homepage doesn't clearly state what a company offers, people leave. If users get lost, they leave. If the information cant be found easily, they leave.
Do you see a pattern here?
One thing to remember when designing for the web is that websurfers tend to have a very short attention span. There are plenty of other websites available and leaving is their first line of defense.
How can I prevent my customers from leaving? A great place to start is by improving the sites usability.
Usability is defined by five components:
LEARNABILITY: How easy is it for users to accomplish basic tasks the first time they encounter the design?
EFFICIENCY: Once users have learned the design, how quickly can they perform tasks?
MEMORABILITY: : When users return to the design after a period of not using it, how easily can they reestablish proficiency?
ERRORS: How many errors do users make, how severe are these errors, and how easily can they recover from the errors?
SATISFACTION: How pleasant is it to use the design?
If you are interested in learning more about usabilty- here is a good resource: