But basically it divides development into three areas the Model (Database access and business logic), View (the presentation) and the Controller ... the interface between the two. (Very rough description).
The servlet can generate HTML, but it is cumbersome, because it involves writing lots of print statements... etc etc. Usually, you think of the Servlet as the Controller. It will receive a request process it... Call BusinessObjects to perform the business logic (access databases, make decisions...) and then forward the results to a JSP which will do the presentation. It is the Controller of the whole process. It should be relatively simple....
Hopes this helps.