Java Servlet is used for Server Side programming for developing Web Applications. It easily employs Database Connectivity. We can also use JSP however it cannot replace a Java Servlet.

Java Applet is an application designed to transmit on internet to execute on java compatible browsers. Java Servlet is a server side program used to provide services to clients.

Networking is a basic action. A servlet in Java is a single part of networking, a single task.

Yes you can but it is not required. A Servlet is nothing but another .java file and all rules that are applicable to standard Java classes are applicable to them.

Note: Even if you write a constructor in a servlet, it will not get executed.

No. Javascript code can be present inside a JSP but not inside a servlet. A Servlet is a pure java class.