David Allen's "Getting Things Done" in one graph
On the horizontal axis, we have the number of “things to be done”. Things may refer to any discrete task that needs to be acted upon.
On the vertical axis, we have the number of “things in your head”.
On Model A, represented by the blue line, we keep everything that needs to be done in our heads. It follows that, the more things we feel that need to be done, the more we have to keep track of.
Unfortunately, there's a limit to how many “things” we can reasonably remember at a time. This is represented by the horizontal gray dotted line. As we approach that, we reach the limits of how many things we can have going on by following Model A.
Allen claims that much of the stress and anxiety we feel is due to the “open loops” we have going on in our heads -- meaning the number of things we haven't completed and need to keep track of. According to Allen, Model A would be very stressful and, on top of that, -- since at any given time we can't do anything about most of the things we've decided to put in your mind -- not conducive to make progress towards anything.
So the Model B is proposed, represented by the green line. Allen advocates that everything must be kept track outside of our heads. We must use journals, notes, calendars, for remembering -- besides, those are much better at that anyway. This frees up our minds to just the “things” of the task at hand, the work that you are currently tackling.
You see that for model B, it doesn't matter how many things need to be done: if you can trust that you've put every tasks into a recorded medium that you will review later -- or that will let you know at the appropriate time, such as a calendar --, your mind can let go of remembering anything unrelated to what you are thinking about.
In short, this is Allen's Getting Things Done (GTD) model: Keep every discrete task recorded outside your mind, rely on those external tools to do the remembering for you. Set up reminders to periodically review those records to ensure you are on top of things. This allows you to let go of any concern such as “Am I forgetting about something?”.
The only thing you ever need to remember is writing it down into your system as soon as you think about it.