power amp/pre amp input and output impedence question

Hello, im in the process of researching a new system, low and behold my limited knowledge has led me to try understanding the input and output impedences to better assemble my new system as to the power amp and preamp.

in all of my years as a Hometheater guy I've avoided such topic because with "most" HT gear it all works together just fine.......

Hear I am many years later "catching up" on terms I should know, I was gonna just google and read, but most of the time well versed members can get to the meat better than random google replies

Thank you KG