Whit the ServletConfig object, the container can pass information to the servlet class, for example the init parameters in the deployment descriptor, and a reference to ServletContext. Whit the ServletContext object the servlet can communicate with the container and, for example, can have access to log files.
It is better override the init() (without arguments) method in your servlet class. The init(ServletConfig config) in GenericServlet call the init() method when finish.
If you need a reference to ServletConfig object in your init() overridden method, you can use getServletConfig() to get a reference to it.