[split] Questions about flashing new firmware

[HappyUser]

I am stil learning a lot here about android, but I havn't tried flashing yet - although reading about it... I still don't understand how much I can get out of a Nandroid backup if I choose to flash a custom or stock rom on my S5...?

Do I wipe everything on the phone when flashing a new rom, incl SD cards?

Do I choose to "restore" my backup after flashing a new rom, and what happens then? Will the backup then install the old rom again, and fuckup the newly flashed rom?

If I choose now to install/flash a new rom on my android device and it went bad or the rom sucked, could I then restore everything back to normal with my nandroid backup that I just made?

I'm sorry for all the questions, but I really want to understand all this android...

[pufferaway]

To become familiar with all of it, take a look at this [Only registered and activated users can see links Click here to register]
There's very detailed info, but it's not a difficult read and there are multiple links within the article to dedicated topics.

Best wishes!