doGet is called in response to an HTTP GET request. This 
happens when users click on a link, or enter a URL into the 
browser's address bar. It also happens with some HTML FORMs 
(those with METHOD="GET" specified in the FORM tag). 
doPost is called in response to an HTTP POST request. This 
happens with some HTML FORMs (those with METHOD="POST" 
specified in the FORM tag). 

Both methods are called by the default (superclass) 
implementation of service in the HttpServlet base class. 
You should override one or both to perform your servlet's 
actions. You probably shouldn't override service().

doGet method will append data to URL while transferring 
data to the server.

doPost method will not append data. It is considered to be 
secured. The data will be inside the body of request and 
response.