My take on it is: A bounce where a visitor turns up at your site and either clicks back or closes the browser without navigating to another page. Total page views will always equal one. A drop-out refers to a given process (say a purchasing cycle with adding things to a basket, checkout and actually confirming the order as examples steps) and the % of people who fail to get past a ceratin point. So for example you may get 100 people add something to a basket but only 50 people go to the checkout so your drop-out rate at this stage would be 50%. If 25 people drop-out without confirming the order the rate would be 25% at this step (you tend to have rates as a % of the people who started a process NOT how many people reached the previous step). Just my 2 cents.