Getting Things Done, or GTD, is a time-management methodology. It’s basically just common sense, but I’ve used it for some time now to keep track of jobs and projects.
I first encountered GTD through its various implementations in Emacs org-mode, it seemed like a sensible system to adopt so I gave it a try. Note however that GTD is not a computer based system: I use Emacs because I use Emacs. Beginners are advised to start by using a paper based system; you can use Evernote just as well.
The following is just an overview. To find out more, read the book: Getting Things Done by David Allen (paid link)