Getting Confused About Something "Simple"

Hi, I hope you’ll indulge me as I ask a “simple” question my mind can’t quite figure out. I know the pieces of this are documented in one way or another somewhere, but could someone give a simple answer (a couple sentences each) describing the interaction between:

1. A post
2. A page
3. A template
4. A category

I’m trying to figure out which template I need to adjust in order to make a given page (my home page, my blog page, or another menu page defined by a category) change.

Thanks!