Looking at the API, we see that ServletContext init parameters are "context wide" - in other words, apply to the entire "web application" - the api says they are created with a context-param entry in web.xml as a child of the web-app element. (I can't say I have ever seen them used, but they look useful.)
Whereas ServletConfig init parameters are specific to the
servlet and come from the init-param tags in the web.xml section for a particular servlet.
Bill