Usability means that the people who use the product can use it quickly and easily to accomplish their own tasks
Usability is an attribute of a product, as is functionality and performance.
Functionality - what the product can do
Performance - how fast it can do it
Usability - how easy it is to do it
The Cost of Bad Design 10 times / day @ 60 sec / time = 60 sec / day 25,000 users 416.7 hrs / day 92,917 hrs / year $5,110,417 / year Observed during a usability test: Because there was no visual confirmation that an action had been performed, users checked the database to see that the action was completed successfully