The rules for this system can be found by clicking here.
Brief Summary
The idea behind Autofocus (as it was originally called) was to provide a system which was much more effective and intuitive than Do It Tomorrow. The aim was to be able to enter all the tasks which arose or occurred to one without any form of pre-selection into a single list and to let the system do the sorting - or more precisely for the system to provide a framework which would enable one’s intuition to do the sorting. The list was written in a notebook, with each page of the notebook forming a closed list. Each page would be worked on in turn until no further tasks felt ready to be done. The important concept of dismissal was introduced which happened when one was no longer willing to do any of the tasks on a page.
Advantages
The system was extremely simple, and was very conducive to working little and often. It reduced resistance considerably and could produce an almost addictive feeling.
Disadvantages
There was a strong tendency for easy tasks to be done at the expense of more difficult ones. It was also poor at handling urgent tasks and tasks which needed to be done in a particular order.
Conclusion
I and many others love the ease of use of AF1. Most of the following systems have been attempts to keep this while improving the focus on important and/or urgent tasks.