Probably the book/system by David Allen. It has a cult-ish following. The google results aren't very vague about it:
https://www.google.com/search?q=getting+things+done
I tried the book. It seems nigh unreadable due to fluff and the author's ego. (I think some reviews say the first edition is better.) However, the system seems good for what it is in principle. (It looks like a well-thought out to-do list system with things like pruning the list not left to chance. It's not a miracle or a silver bullet.) I learned with this:
and use Mac's OmniFocus for the lists, as such.