Both technology is use for creating a web application. The have different purpose. For instance the servlet is better use for controller because it will be hard to maintain the look of you web page if you create it using servlet. For instance if you need to change the layout of your page you have the edit your servlet then you need to compile it again.
While JSP is use for displaying data, or the content of you pages. All logic to retrieve the data, etc should be delegated to the servlet. Your web design will be all in the JSP page, this also means that a web designer who might don't know what servlet is will easily alter the look of your page.
So again you can use both technology. Just be sure they are in the place where they work best. Here is an example of a simple how to write a web application in MCV [
JavaRanch FAQ : Code Barn Simple Mvc]