servlet context:servletcontext is only one web web 
application object
servlet config: servlet config is multiple object in 
webapplication

Servlet config is one per servlet and servlet context is one for
the whole webapplication. 

Servlet config is used to pass deploy time information to servlet
where servlet context can be used to get information about the
container,server.

YES,There is some difference,when we deploy the web 
application the web container will create the Context 
object and when we load the servlet class then container 
will create config object,so for each web application web 
container will create one servlet context obj and for each 
servlet web container will create servlet config object.

Generally we use this objects to avoid the hardcoding,we 
declare this variables in deployment descriptor,we declare 
context parameters under web-app tag so that all servlets 
can use this parameter values ,and config parameters under 
servlet tag so that only specific servlet can use this 
parameter values,in simple way context object is like 
global object and config obj is like local obj.

Yes, some difference is there b/w servletconfig , servlet
context and session.
ServletContext:This is an interface created by webcontainer
based on class provided by webcontainer wendor,the
webcontainer will be create ServletContext Obj for the whole
web-application.
ServletConfig:This also an inteface and created by
webcontainer based on class provided by webcontainer vendor,
webcontainer creates servlet config obj forevery resource
that is available in webapplication.
session:session can be used to develop stateful application.

You can get reference to ServletContext through ServletConfig. HttpSession does not provide a reference to ServletContext object.

To get a ServletContext object use

getServletConfig().getServletContext();

or you can call directly getServletContext() bcoz GenericServlet class implements ServletConfig interface also.

The ServletContext returned from ServletConfig and from
HttpSession are the same instance, the one instance that
exists while the web application is running. You can confirm
it with a debugger.

The main differences though are that you get a reference to
ServletConfig when in a Servlet and you get a reference to
an HttpSession from an HttpServletRequest.

You can get a reference to a ServletContext from a
FilterConfig when in a Filter.

If you are not in a Servlet and don't want to create an
HttpSession through a call to
HttpServletRequest.getSession() to be able to get a
reference to a ServletContext then create a Filter which
puts a reference to the ServletContext in a request attribute.

YES.

ServletContext(): To communicate with the Servlet Container.
                  Eg: What server to running on the 
                      Environment.Then when sever is 
                      started,who is login the server all
                      the information to store the 
                      "Event log"file ie log4j.  

ServletConfig(): Pass the configuration information to the 
                 Server to the servlet.  

Session():       Session is a Object to track
                 the user interaction with
                 the web application across multiple
                 HttpSession.