Like Servlet, JSP is another server side technology which is used to develop dynamic web application. Basically it represents the presentation logic of the web apllication.
JSP pages are easier to maintain than Servlet. Servlet adds HTML code inside Java code while JSP adds Java code inside HTML.
JSP is an extension to servlet because it provides some more functionality than servlet such as expression language, custom tags, jstl etc.
A JSP page is nothing but of some HTML tags and JSP tags. In JSP page we can separate designing and development. I mean JSP separates presentation logic and business logic. Designing and updating JSP pages are very simple.