Q: Iphoto and Photos: do they share or duplicate files

It seems iPhoto was the main photo software before, and now with my iPhone 6 it is Photos.  not sure when the transition occurred, but i seem to have photos in both.  I don't see iPhoto on the IP6

Both programs are on my MacBook Pro.  My primary question is,  Do both programs share the same photo files or are there two copies of all the photos on my MacBook?   Is there a way to sync the two programs or make them use the same photo files? (i fear there are two sets of photo files, and i want to be conservative of space on my laptop.)

Secondly, can someone recommend a clear discussion of these programs, how they work and store files, and import approaches?  I am fairly computer savy, just don't spend a lot of time with photos and don't have the patience to dig into them that much and find Apples attempts to "protect me" from understanding what is happening to be extremely frustrating.  their idea of "intuitive" and mine seem to be at opposite poles.

Frankly, i find Photos less than intuitive on my MacPro.  seems OK on the iPhone.  Am still trying to figure out how to easily copy new photos from the iPhone6 to my Mac where they will get backed up.  I rarely want to go the other way.   iPhoto seems more logical, but i think it must be being phased out.  Perhaps they are making this difficult to force me to use iCloud, which i don't want to do.  I see it only eating my battery and minutes trying to sync all the time.

Thanks in advance for your advice ..

tom