Usable design is focused on making it easier for your visitors to accomplish their intended task or goal.
Persuasive Design on the other hand, is compelling that person to want to either get to a place of action or to perform a given task once there.
Without persuasion, making something easy to accomplish doesn't necessarily translate into making it something people actually want to do.
“Making a website easy for visitors to use makes them more proficient users, but does not necessarily make them customers.”
Think of it as "can do" versus "will do" or "should do".
