Difference between Activity Context and Application Context

Question

This has me stumped, I was using this in Android 2.1-r8 SDK:

ProgressDialog.show(getApplicationContext(), ....);

and also in

Answer 1

I think when everything need a screen to show ( button, dialog,layout...) we have to use context activity, and everything doesn't need a screen to show or process ( toast, service telelphone,contact...) we could use a application context