It might seem that a personal organization system which requires that you begin by pulling together, in one spot, everything in your life which needs action, or which you want to do or even exercise vague fantasies of doing, is the opposite of simplicity.
Actually the Getting Things Done (GTD) approach is much less complex than letting a wide array of nagging unfinished tasks continue to spontaneously distract you at inappropriate times, and disrupt your schedule in a manner from which it's difficult to recover.
The system is based on the premise that the unaided human mind is a terrible organizing tool. Without some external and complete tracking system, tasks will pop into your head at times you can't do anything about the issue at hand. The mind is not very good at evaluating the context. For instance, it's no use thinking about a phone call you forgot to make if it's 2 AM. It's a waste of mental energy. Likewise mulling over tasks requiring a computer while you don't have immediate access to one is pointless. You should be focused on some action you can actually perform in the context in which you find yourself.
So the GTD system divides workflow into five steps:
1) Collect (or capture) the individual items you're working with
2) Process the items (figure out what the item is, and what should be done about it
3) Organize the results of the processing
4) Review the organized items periodically
5) Take the actions needed
What this looks like in overview is that you pull everything together and evaluate each item. Items which take less than two minutes get done on the spot, other things might be delegated to someone else, or deferred for a later action. The deferred items get organized onto lists. Some of the lists are by context (for instance, things to do when you have access to a phone) and some are by project. Every item has a "next action" associated with it. This is key. Another premise of the system is that items on todo lists which don't spell out a specific next physical action are incomplete. The lists are then frequently reviewed and updated. Finally the action steps are done, along with an evaluation of any needed next steps.
I've found in my increasingly successful (but not quite victorious) attempts to get the system up and running that the collection stage is the most difficult. Once thought is brought to bear on any individual item, it's pretty easy to figure out what list it belongs on, and to come up with a next action step. Making sure that your system is initially complete is the hard thing. Then, like any system, maintaining it becomes an issue.
Leo Babauta, who publishes the excellent blog Zen Habits, has developed a modified version of the system called Zen to Done, and has described his system in the book Zen To Done: The Ultimate Simple Productivity System
My notion of the Holy Grail of Getting Things Done is to have a complete set of lists, organized by context and project. Every currently known project or task in my life is there, along with sets of actions I need to take. When an emergency, new task, or interruption occurs, I just incorporate in into the system and take the appropriate steps without having my life thrown into disarray, even in the short run.
Of all the organizing or "time management" systems I've tried, GTD has come the closest to its promise.