mod creating/editing question

[homsikpanda]

all the guides i've read (3-4) left out this little question that's been nagging me a bit...
when i edit a file, or create a mod that does this or that, when i save it does it save to a different file location? or overwrite the original file? if it saves to a different file, where does it save to?
if it overwrites the original, how do i get things back to normal?