ServletContext is the over all web application. These include the the parameters you list in the web.xml in the context-param element. (outside any servlet elements you specify) There is a single copy for any and all servlets and jsps running in the web application. Additionally, any Attribute you add to the ServletContext is available to them all and there is one and only one.
ServletConfig is the configuration for a single Servlet or
JSP. These include the init-param you list within the servlet element of the web.xml. ServletConfig doesn't have the ability to set or get Attributes.