After reading through pg 71 and 72, I still don't understand what is a "category" and why it's useful to set or define in the manifest.

Can someone clarify?

Thanks.

That is more than I can claim, but it appears to be an adjective which is applied to group categories. The first and last (p73) sentences of the section seem to be the "why" and the rest is just "how". Have a look at the reference for the Intent class here : http://developer.android.com/referen...nt/Intent.html

And the very best of luck!