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For knowledge workers, there is a lot of work to do defining our work before we can start actually doing our work. The 5 phases of workflow is the GTD model for capturing, defining and doing our work.
A mechanism to remind yourself of something at a given time in the future. When the time arrives, the contents of the tickler file are moved into an inbox for processing with all other stuff.
The practice of setting aside dedicated time (typically a couple of hours) to make sure your system is clean, current and complete. David Allen regularly cites this as the critical success factor for GTD.